Just let him go through the stages, everyone has to and there's really no way to skip them. The absolute best way in my experience and others' is to cut all ties, get rid of all things that remind you of them, and just let yourself breathe and try to forget about it. The best thing for them to have is a shoulder to cry on, but if you treat it like a big deal (god she's such a cunt! you deserve better! the guy's not even cute!) it will stay a big deal. Not to say don't let them talk it out with you, but just don't build them up more than they have already or they will crash even harder. There will be lots of ups and downs but what they need is time, even if it is a super cliche.

Being cheated on is one of the fastest ways to feel worthless, I can only imagine what it must feel like when engaged. Sorry about your friend.