Chickens can develop what we call flock immunity. They may have a disease and be carriers that can infect other chickens but will not show symptoms themselves. Coccidiosis is a good example but there can be others. If a chicken is exposed to a certain strain of the protozoa that causes Cocci for two or three weeks they develop an immunity to it but can still pass it on to other non-protected chickens. Whether the chickens are vaccinated against Cocci or not they may still be carriers. That could be your flock or your friend's flock. The vaccine may not protect against all strains of Cocci.
Quarantine is a very effective tool when done properly. Most of us can't do it right. Chicken diseases and parasites can spread by sharing food or water dishes, pecking where others have pooped, by vectors like mosquitoes or grasshoppers, or just on the wind. You can infect one flock by wearing the same clothes (especially shoes) when you manage them or or use the same bucket to carry feed or water to them. The more you isolate them the better your quarantine, even a partial quarantine offers some protection, but most of us aren't set up to really manage that.
I keep a pretty closed flock. I do not vaccinate any of mine for anything. The only chickens I add are just-hatched chicks from established hatcheries or from eggs I hatch, my own eggs or from a neighbor's eggs. I do not wear the same clothes to manage my chickens as I wear to a show. I do not show my chickens or sell them at a swap or auction. I try to minimize the exposure to diseases or parasites. It's not a 100% guarantee, my flock has at least one strain of Cocci. It was just in the ground or maybe a wild bird pooped in the area.
If your friend keeps a pretty closed flock and that flock has not been exposed to outside birds for a couple of months and your friend would recognize a disease or parasite if they saw it, that other flock has essentially been in quarantine. Probably a better quarantine than you could do but those are some big if's.
If you bring that boy into your flock it is possible he could bring something bad in. It's possible your flock will give him something. If you bring in hatchery chicks it's possible your flock could infect them. Many hatchery vaccines take two or three weeks before they become effective. You need to isolate them from your flock like a quarantine for those vaccinations to be effective. If you brood them in your house you're probably OK. If you brood in the coop like I do the vaccinations may not be effective anyway.
To me there is no clear cut answer. The safest way to add a new chicken is as a newly hatched chick from an established hatchery or hatch some eggs yourself. The most risky is from a swap or auction, though many people regularly do that. If they do bring something in it will most likely be more of a nuisance that can be treated like like mites or worms than something that will destroy your flock. But the risk exists.
I don't know how much risk there is from your friend's flock or how much it would devastate you if something bad did happen. That has to be your decision.
A reasonable thing to do, and just watch for something developing. Practice ‘social distancing’ from the other chickens and wear a ‘mask’, ie covering or some way To minimize picking up a germ on your person. Wash your hands between your chickens and the roo. we all should be washing hands after messing with chickens anyway. Nothing earth shattering, fairly routine. Besides a quarantine would give you opportunity to interact with him one on one and get him accustomed to his new person and surroundings. See his likes and dislikes. Before throwing him to the ‘wolves’. 
Thanks!
