I would keep them together as long as possible!
I would also try talking you're friends into letting the puppy stay inside, until the puppy is at least 6-8 months at the very least! There are so many dangers outside for a young puppy and being out there alone at this young age when he's use to being with his brothers/sisters has to be so scary. Someone can easily come in the yard and steal the puppy or some type of wild animal could attack it.
I'm not sure where you live, but the summer heat can also take a tole on such a small puppy. Please try to convince you're friends to crate the puppy inside and keep them together until 8 weeks at the very least, !0 weeks would be ideal.