Reddit mentions of Cowboy Magic Concentrated Detangler and Shine Great for Pets and Human Hair! (16 fl oz (473 mL))

I would post on /r/findareddit, asking for a hair advice reddit.

Be as descriptive as you can of her hair texture - In the world of hair care, there can be pretty distinct differences in techniques and products depending on texture.

My only experience is with horses - I've successfully detangled manes and tails many times. The product I've used is Cowboy Magic Detangler & Shine. It's an oil type "finishing" product, smells nice, works really well, and a good quality product that overall helps hair. If you search Amazon for "hair oil", you'll see lots of similar products for people.

As for technique, the one thing that's really helped me is just leaving it in overnight. Of course, with horses, you don't have sheets and blankets and pillows and mattresses to consider! So maybe with your mil, apply in the morning, planning to do detangling in the evening. Anyway, main point is, when I would saturate, and then immediately go to work with a brush, I wouldn't get very far. But when I would saturate, then come back the next day with a brush, I could get tangles out pretty easily.

Anyway, hope that helps, good luck!

Edit -

I'm going to second /u/justmerc 's suggestion - My son had long hair when he was a kid, and I would do that very thing, just slather on the conditioner in the bath, comb it easily, then rinse thoroughly.