Reddit mentions of LIBO 125KHz Smart ID Key Tag Card RFID Access Control Key Card Proximity EM4100 TK4100 Keyfobs Keychains NFC Token Badge Holder (Mix Color, Pack of 10)

So this is rather long and I tend to be a little winded but here is an explanation and if you wish to take a chance with spending a little money you can try to see if you can register a new fob and learn a little about RFID.

If the lock just reads the built in UID the Unique Number that is on each fob then it could be that the system uses an administrator/master fob to register other fobs.

How that works is that if you put a master fob up against the lock and then within so much time another unregistered fob is put up against it then that fob is registered by the lock to open it.

Your friend might have a fob that was programmed as the master/administrator fob so that when his fob was put up against the lock and your fob was put near the lock within 5-10 seconds which since it was on the same key hain that would be the case and its UID was registered to open that lock.

There are several variations of programming and not all locks work the same.

The best systems and especially the newer systems will use a keypad or a program on your phone and not a fob. This situation is not yours unless a glitch happened to allow your fob to be registered.

The middle systems will have a master fob but that fob will not open the door too. This situation is not yours.

the simpler systems will allow a fob that unlocks the door to be a master fob too. Which is what I am talking about here and you can see how that can lead to other fobs being made keys that open that lock.

Here is a video of either a middle system or the simple system where you have a master key. Hard telling if master key actually opens door because they don't show it other than using it to program. In the video you picture that your friends key was the white card. The reason they used a card was to differentiate the master from the unlocking fobs. They could of easily programmed one of the blue fobs as the master. The white card is just different packaging of the fob and they are identical other than looks. So you could see that if his was designated a master how by having yours on the same keychain could of registered it.

https://youtu.be/N7tLpwxlUCg

Now also realize that NFC and RFID are not interchangeable.

all NFC cards use RFID to work but there are other forms of RFID out there besides NFC. NFC is smart it writes stuff to the card.

Another form of RFID are the Token fobs usually running at 125 Khz. they are considered dumb. In this case there is just a unique number on the fob that the RFID reads and if that number is in the locks database it opens the door. If that number is also a Master then it will allow you to program more fobs. In the simplest form any fob put up to the lock within 5-10 seconds of the master being used and held there for a few seconds will now register that fobs token(unique number UID) allowing it to unlock the door. Some work like in the video where you have to touch the master again to the door to close out master mode and have the new fob take while others might just timeout within a certain time and register any fob put up to the lock before it times out. Both ways your fob could of been accidentally registered to that lock.

If you really want to know more take a picture of the fob and of the lock you hold it up against. post it in a comment. I might be able to give you better answers. If you are worried about security of doing this really don't as no one can do anything unless they have the UID from the RFID on it. so if your Fob actually has the UID number on it which some do and some don't blur that out or take a picture of the side without it. If you also take a picture I might be able to find the exact programming instructions. Include any model or manufacturer info on the lock in the comment as IDing by looks might be hard.

Second is that if you go on Ebay or amazon you can for super cheap have some fun figuring out if you can program a new fob like in the video if your friend's fob is a master and learn some things on RFID by going online and buying cheaply some 125Khz Token Fobs.

Here are two links one to E-Bay and one to Amazon. Ebay you can get 2 fobs for $.99 The big thing is shipping from China they will get here sometime in December or later. If you are that curious and like to learn and a little more money does not bother you, go with the Amazon one for faster delivery. Disclaimer is that I can not guarantee this will work and I also have no affiliation to these sellers, I just picked the first cheapest fob that sorted out on ebay and the first cheapest fob that was in the US so you will get it sooner. So 2 fobs for $.99 cheap but will take a month or so to get here or 10 of them for around $10 with shipping, way more than you need and with shipping they are $1 each compared to $.50 but will get here on a regular quick schedule. Note you might have some cheaper fobs pop up on Amazon as advertisement and only two of them but don't get them. Some Fobs are sent out blank with no UID that you can take a UID from another Fob and clone it to the new one so you don't need a master. You need a fob like I chosen already programmed and this was the cheapest and least amount of fobs that was located in the USA. There was a few 2 fobs that were programmed and selling for about $2.50 but they were also shipping from China like the E-Bay one.

https://www.ebay.com/itm/5-10-20pcs-125Khz-RFID-Proximity-ID-Card-Token-Tags-Key-Key-fobs-Access-Control/162935957346?hash=item25efbd6762:m:m8TlK80YQTR_agg9fXx6PSQ

https://www.amazon.com/LIBO-Smart-Control-Proximity-Keychains/dp/B07GLFYSZC/

So then once you receive the fobs you will go to your friends door and do similar to the video. Hold your friends fob up to the door when it activates hold one of the new fobs up against the lock for several seconds and hopefully the lock might beep or light blink, i if it does you can pull it away otherwise just hold it for a 3 or 4 seconds against the lock then pull away. Then first put your friends fob up against the lock again in case it uses like the video a tap to put into master mode and a tab to register and take it out of master mode. Then try your new fob and see if it opens the door.