I don't know your legal requirements about housing a rooster, maybe something to consider. The issue with bringing in any outside chicken is biosecurity. Are they going to bring in a disease or parasite your don't have or will yours infect or infest him for when you send him back. Do your friends have more than one rooster with their flock? If so, there may be reintegration issues between the roosters.
If you and your friends are OK with the biosecurity and potential reintegration issues I don't see a big downside.
I understand the allure of hatching your own eggs but could you just get hatching eggs from your friend?
My sister offered to let me borrow her rooster and I said the same as @Ridgerunner. I didn't want to quarantine him that long, let him use my girls for a while, then have to go through the quarantine again for his home flock AND have to fight for his position back w/in that flock... at the time my sister had two roosters.
Apart from the biosecurity standpoint, that's absolutely fine as far as throwing him in with your hens for a week and taking him back out. That's what many breeders do, the rooster comes to visit, does his job, then is taken back out again. But you have to weigh possible disease spread in your decision. If I were going to do it, I'd at least deworm him the first day I had him and keep him in quarantine at least 2 weeks.