Most Helpful Guy
I suppose you could dance around the issue of helping your friend get back together with him for a while, and put it off, until he gets back. In the meantime, try buying a pregnancy test, and see if there even IS something to worry about there, first. When he gets back, you can talk about everything. You can talk about where you two stand relationship-wise. You can talk about whether or not you want to be public about it with your friends. If you're pregnant you REALLY need to talk about that, and if you're not, that's one less thing to worry about.
Still, buy some time on that thing about hooking him and her up, before you tell her what's up. Buy a pregnancy test, and be sure. Talk things out with your guy, and then you can decide what to do about telling your friend. If you two stay together, you're going to have to tell her sooner or later anyway. If you're pregnant, you'll have things you need to tell other people one way or another.
Don't worry about it too hard until you know there's something to worry about. Keep a cool head. Work this out as one item on a list, at a time. You can do this. Believe in yourself.