Close friends ARE part of my family. Id send my dbf to his be in his friends and wish him well, and Id go enjoy my sisters wedding. My sister would totally understand. So would my family, since they know how close we are with our friends.
I think being asked to be in a wedding is a huge honor, and is reserved for the people you want at your wedding the MOST. So his friend will really miss him if he has to ask someone else in his place. I think he will be missed at whichever one he doesnt attend, but I think your sis would be less hurt than his friend would.
How close is he to your sis? That makes a diff if they are really close.