We have a friend doing something similar. We'll call the friend H. And the guy can be...D (D for ...a word beginning with d that you can probably guess )
He was with D for a year so obviously they were really close. However D always gave us (H's friends) a very bad impressions, he'd stare at you angrily, come over and cut between you when talking to H (because apparently the whole world was trying to take him) and other pretty manipulative things.
Then one day D writes H a letter saying "sorry we can't be together anymore because our views on...vegetarianism and prostitution are just too different". WHAT?! H is devastated.
The next day D rings "I love you". A few days later D rings "Happy one year anniversary. its a shame its like this" :| WHAT?!
As much as we helped H through it and concvinced him he was better off without him, in the long run you can't do anything about it. At this point we all hate D but then recently H has started going out with D again saying "i know i'm being an idiot, and i know i'm going to get hurt, but i want to do this" and unfortunately we have to respect that. D decided to "be there" for H after our friend died and would often take H away in private and not let us speak to him . He's a jerk but we've all made it clear about that and H knows that too. But it's still his decision and so as a friend, you kinda just have to grin and bear it, and then pick up the pieces when it all goes wrong. *sigh...*