"He probably actually needs a smaller one than a larger one. If the condom is too large it will slide off more easily than if it is a snug fit."
Actually, his "friend" might be too big for the condom, and that's why it's riding up. Imagine when people wear a very tight shirt, and the shirt is too small. The shirt will ride up over the person's stomach b/c it "rolls up" (has something to do with physics). So it is with condoms and the guy's "friend"; the same laws of physics is at hand. Therefore, #24980 should try different size condoms until he finds the one that fits correctly.
And the guy(and girl) must make sure it is put on correctly. It should be rolled all the way down to the base of the penis, and it shouldn't be too tight nor should it be too loose for maximum comfort and performance.