Reddit mentions of Beginner's Croatian with 2 Audio CDs (Hippocrene Beginner's)

A family friend of ours married an Australian woman a few years ago, she didn't know a lick of Croatian when they met. Fast forward 4-5 years she speaks better cro then some people who were born there and that's no exaggeration. Also, we have an African guy who comes to our local Croatian based soccer club in Australia. He was adopted at a young age by Croatian parents and he speaks fluently. You already have an incredibly valuable learning resource right in front of you, take advantage of that. When you get some basic words/phrases under your belt, try and conversate with your mrs even if you don't sound good. This book is pretty good if you wanna know the grammar and its pretty up to date and teaches colloquial words. Listen to some online radio at www.tunein.com and even if you don't understand it all you'll pick up on certain words that'll stick in your mind then look up in a dictionary or ask your mrs. Watch movies as well if you can. The trick is to immerse yourself in the language. Kinda blabbing on here but you really do have a lot of resources to learn considering its not a widely spoken language compared to Spanish or German. Good luck with your endeavors.

Your choices as a foreigner to get going are between titles that contain either "Croatian" or "Serbian" (if you find older material, it'll be likely advertised as "Serbo-Croatian"). "Bosnian" stuff is still pretty much restricted to this book which might actually be overkill as a total beginner learning independently.

The most important thing is to get started with a decent course. Teach Yourself Serbian, Beginner's Croatian and Beginner's Serbian are good starting points if you're really motivated (FWIW, I've used all three). You could also get a taste of the language(s) in everyday life by watching short videos involving Croats and Serbs.

If you learn the basics of any of Bosnian, Croatian or Serbian, you'll be able to start communicating with him. If he plays along and speaks to you in his native tongue, he might adjust somewhat by using fewer features/words characteristic of Montenegrin or speaking more slowly or clearly and using a slightly more formal register than he would when he's with his friends or family.