Technically poison ivy is not contagious, it is only spread through coming in contact with the plant oils. If your friend has had it for a time, it is usually fully bonded to the skin in a short time, the spreading that is ofter seen is usally either due to re-exposure or different levels causing slower reactions. There should be no risk for spread to anyone in the tub.
Personally I would be more concerned if your friend has open sores, could be an opening for serious secondary infection, I know we all like to think are tubs are well maintained and no extra bacteria living in that nice hot water, but for there safety I would be very careful.
Granted I am not a physician and do not claim to have the answer, just wanted to share some information.
http://poisonivy.aesir.com/view/welcome.html This is a great poison ivy site and the message board is monitored by MD's and the other people there are very helpful.