What shall I do?

My former flatmate is here on an official visit and instead of staying at the company's guesthouse, he has been staying with me. I won't say it is because he can save a lot of money that way, but I won't rule it out. It looks like he can never forget this girl who was working here with us. He used to be in her department and he tells me he was quite close with her. Long story short, he is obsessed with her, she doesn't want to do anything with him, she is planning to marry her boyfriend, which our hero can't digest. He has been complaining to me for the last few days how he can change her mind and convince her to marry him if only she would reply to his deluge of emails. He tells me how every girl he has worked with have thrown themselves at him and she was the only girl he has ever pined about - 'why can't she realise that?' He takes this as a personal failure but can't see it that way. This seems to be my role since seventh standard - listening to "friends" cribbing about women who don't want to do anything with them, and they weren't exactly on my good side to begin with. The only good thing that came out of it seems to be being privy to their dirty secrets, their weaknesses, being dined at their expense, and ofcourse, the fun of seeing them embarassed at the least. But with this guy, I am afraid he might harm the poor girl like we see in many news reports, especially considering that he is from North India; that too, from Delhi.