Ok, you said it was his girlfriend of 3 weeks, correct?
You are young, and even though a 3 week relationship may have felt like the longest time, it really isn't. He will be able to move on fairly quickly from this one, unless he has some serious attachment problems... lol
I have been with my girlfriend for just over 5 years now, and she was my buddy's ex of a 1 month relationship. He and I are still just as good as friends and he is even having a kid with his new girlfriend (yes, it is 5 years later, but still).
So, all I am saying is that, it could be a good move for yourself. I got together with my girlfriend and started clearing up my life of a lot of problems. She is now the woman I plan to spend the rest of my life with and she just started out as my buddy's ex.
You never know =)