I'm very worried about my friend! Does it seem like her boyfriend became faithful?

Ok so long story short, my friend is dating a guy who was in my high school. (Don't worry, no jealousy, I already got a bf anyways). They've been together for quite a while. Thing is, I already know lots about her bf and have been involved in situations with him and his exs (when he was dating them). He always cheated on every other girl he dated, he would have with girls who didn't want a relationship, kinda like FWB. He wasn't a faithful person, he got his previous ex pregnant at 17 and he was 18, when they were dating (I was already friends with her). But he never told his new gf(my friend) that. She doesn't know he has a kid. Anyways he's dating her now and for some reason, he's being quite faithful. I also constantly see him writing stuff to her like "Omg i love you honey, i love you, i love you!!!" Thank god they haven't had , I wouldn't wanna see my friend get preggo. Anyways I still got some kinda bad feeling. How come her bf (who was always a cheater, liar, manipulator, etc) in the past, is now way different? And he quickly changed into this in like 1 month. He's being so proper and innocent to my friend, and that's a good thing but at the same time, it confuses me. How can a no good cheater suddenly turn faithful in 1 month. I got this bad gut feeling inside. How come he's being so innocent, happy, and exciting with my friend, telling her about their future marriage and stuff, when in the past, he was never like this? I'm worried about my friend, I got a bad feeling that something is going on... Any guesses??? :(