I have got to say, I agree with Ellie. DBing is for M's. Dating is for figuring out who is "suitable" for a life long M.
Like Ellie, I wish I would have listened to all the red flags. my ex cheated on me a few times while dating, but he talked himself out of being guilty on "technicalities".
And ellie brings of the excellent point of this not being a "mistake" she cheated with his best friend!
He is so young and still has a life to explore ahead of him. He needs to get out there and not start a life together with a woman who would cheat with his best friend.It's not a good foot to start off on. If they are meant to be together, they will both take some time, grow, and find each other.