Where's Skiploder? He's IMO the expert on RIs on this board!
I would say your friend's husbrandry is at fault if her snakes are all falling ill to RIs. Were the sick snakes showing any other signs of ill health other that the RI symptoms (regurging, stargazing, etc?)
As for orally treating RIs, I believe injectable antibiotics do so much better than oral applications. I would recommend discussing doing injectables with the vet. Also is the 98/99 degrees hot spot temps? If so, I would lower it a tad since that seems a bit too high (IMO). I bumped my one girl's hot spot temps to ~95 when she came to me with an RI and after 30 days of injectable Baytril shots she got cured of it.
As for protecting your snakes: wash and sanitize your hands before coming home and right when you get home. I wouldn't worry about your snakes catching anything though as long as you're strict with santizing yourself after being around the sickly petstore snakes.