She can ask her gasteroenteroligist to do a remicade antibody and concentration test just before her next infusion. If she has developed antibodies against remicade, then things like rashes, joint aches, and fevers are a lot more common. Many labs do this, here is one as an example: https://www.labcorp.com/test-menu/29496/infliximab-concentration-and-anti-infliximab-antibody
I would make sure she is premedicating with tylenol and an antihistamine (bendryl/claritin) just before the infusion. Mention this rash to her gasteroenteroligist and infusion nurse. They might even choose to premedicate her with IV solumedrol (a steroid). Any of these things might reduce the rash, if it is related to the remicade. Worth a try at least!