...all he said was "can I give you a compliment, I love your hair it looks beautiful". That's a sad thing if we seriously can't give people simple compliments like this. There's two things that I think is totally appropriate to comment on: hair and clothes, as long as it's true to the clothes or the hair. (As in do NOT say "wow that dress really makes your ass pop", that would be wrong and sexualized and creepy). Generally just saying "your hair is beautiful", "I love your outfit", "your shoes are really cool" are all fair game and not sexualized. Those kind of comments can make anyone's day male or female. I'm assuming your FIL didn't try to sniff or touch her hair, correct? And didn't go any further when complimenting her? I don't see anything wrong with that. If I have nice hair or look good in an outfit id love for someone to tell me it looks good (cause then I'll keep doing it).

The only thing he did wrong was when you guys ganged up on him at home and he responded "I could pull her if I wanted" now that was an asshole thing to say, but his initial comment? No. Not at all. It was a genuine compliment.