Well, it might be because of the child. Child birth really loosens the walls of the vagina. I think she could go to the doctor and get it stitched up some way and she could do kegels every day. About 80 of them. Give her up to 3 months, she should be significantly tighter. It does not mean she is having sex with someone. I think weight gain has something to do with it, I am not sure but the looseness of her vagina does not say she is cheating.