My bestie asked me a super awkward question and im not sure how to answer?

Well her and her fianc�e have been together basically 3 years (cant keep record lol) and they have a kid together,but she just text me and said he wants to sleep with someone else to prove he is actually in love with her...to me it doesn't make sense...apparently he thinks about it quite a bit and thinks that it'll make them stronger together (or something) she only had her baby 4 months ago so she's feeling super self conscious about herself...she thinks he just wants her to think its ok if he cheats...please help,she hasn't stopped crying since he left...