Relationship Problems

I have a friend who is going through some tough times right now with his partner. He explained to me that they keep having this on and off relationship, they keep taking off the realtionship because she's tired of him keep their relationship a secret. He has put a lot of himself to keep her and the relationship going but to only find her 'cheating' on another guy but he doesn't know exactly what to do. Keeps blaming himself that he did something wrong. Could I get advice on how to help him? Please?