In my opinion this girl is very stupid to of done what she did, she of course will be extremely ashamed in herself not to of told anyone for so long, but i'm sure she is very sure of that already, girls only cry for good reasons and if he says he's never seen her like that, then that must mean something huh?
Everyone makes mistakes & should be given another chance, if they are truly sorry for what they did then why shouldn't they have one, of course people can say 'once a cheater always a cheater' but thats a SAYING, it's not true for everyone is it, in fact it's probably only true in a really small number of situations. Sure, there are people who will cheat and say they're sorry etc but not really mean it, and do it again, but at the end of the day that doesn't mean every person is the same, if someone shows absolute remorse then that must mean they are totally meaningful when they say they wouldn't do it again.
If Adam still loves her, it's hard for those feelings to just be turned off. Loads of you say just go on holiday get with abunch of girls, go out with his mates etc but thats not 24 hours of the day, what about the times when he's at home on his own just thinking about things? He can't spend every hour out with his mates, or playing xbox, etc. If he feels his life is missing something then it must be missing her, and if he's missing her he needs her, and if he needs her he loves her, and if he loves her, there may be a way to work past things.
Its obviously not easy and takes a hell of a long time but if two people have been through alot, and she's been there for him, they are best friends and maybe even soulmates, why shouldn't they be together? It will be hard for him to forget about what she did but like I said before, time will make things better.
She made the wrong decisions, which were terrible ones, but she knows that and i'm sure she feels as low as she can possibly get because she let herself down in such a way, and let everyone else down. I bet the guilt is eating her up inside and she must cry everyday. But if she hasn't let Adam go by now there is still a lot of love there, enough to be together.
By the sounds of things these two people would be much happier in the long run being together than being apart.
Just my opinion.