Generally when talking a handful of birds, in a small backyard flock, I do not promote quarantine, as it is nearly impossible to do in limited space, and if you don't do it right, you may as well not do it. You can have a wreck, but you are talking just a few birds.
You, on the other hand, have 30 birds. Thirty birds in an investment, and in my opinion, a bit to risk. I would assume if you have 30 birds you also have space to do a proper quarantine.
You could get a disease that would wipe you out. You could get lucky and have no problems. If the other person has just 3, chances are she does not go to shows, or auctions or swaps, which dramatically increase your chance for exposure to disease. If she only has 3, there is a good possibility that there are not other chickens near her that could be a pool for a disease. If she has a good set up, chances are these are very healthy birds and will be no problem....... BUT there is a real risk.
Separation is not quarantine. Quarantine is very involved, and involves a change of clothing including footwear, distance apart from the old to the new of 300 feet (I think), and even separate feed bins. If you don't do it right, you may as well not do it at all.
Personally, if I didn't want to risk the 30, I would not take the birds.
Mrs K