Was I selfish to do this?

Ah, you broke the unspoken rule about getting involved with a friend's ex. This is exactly why people warn you about dating a friend's ex. You will usually end up losing that friend and the guy too. It is a bit on the selfish side, but not totally. The situation has gotten to the point where the best thing to do is stop interacting with either. No more calls, etc. Just walk away and be thankful you're not in that situation anymore. There are many other less dramatic possibilities out there. Find them!