I think you'll be ok as they already get on. As with a child, just make sure he still gets lots of attention, as I'm sure you will. He'll probably like having a dog friend as well as you, so just see how it goes.
Will your friend want the dog back at any point? Make sure you are both clear on ownership, as you don't want to be paying for food, vet's fees etc and find your friend's circumstances change and he wants the dog back!