Is he friends with her? Is he member of some groups or does he play Farmville or some of the fish games, etc.?

Not trying to defend him, but I personally often receive random friend requests from people I don't know. So of course, i check their profiles than. If he has some good looking picture in profile, it's quite possible some chicks will befriend him...

So please, don't be agressive about it. Ask him, but in a calm way. The worst you can do is blow it out of proportion.