A lot of people pretend to quarantine. If you do not do quarantine right, you may as not do it at all. It requires a great deal of space, 300 feet between birds, a complete change of clothes, separate feed. Most people with a backyard flock do not have this capabilities. Healthy chickens do look healthy. I have added chickens and gotten lucky. Do NOT EVER add something you feel sorry for. And don't buy chickens at auctions. Those chickens have been exposed to God knows what. I belong to a Facebook page, and those people add willy nilly from auctions and that makes my blood run cold.
So in your situation, I would add the birds, not quarantine, and not worry too much.
BUT it is a real possibility, that those healthy birds, could bring in something, your birds have never seen, and wipe your current flock out. Realistically, if this is your friend, ten to one, you have already visited her chickens and could have very easily already tracked diseases back to your flock. In valuable flocks, people would wear shoe coverings, so as not to track in possible disease. Or not allow visitors at all.
If losing your current flock, would be a financial loss or an emotional loss that would be difficult to deal with, then don't add strange birds. Raise up replacement birds from day old chicks.
Mrs K