Welcome to Bladder Cancer Canada. Dental issues can arise while undergoing treatment.
When I was doing BCG treatments I found my gums were more sensitive (gingivitis flare up) and that was alleviated by going for extra cleanings and being very diligent about brushing and flossing. I also used Listerine mouthwash as it is made for eliminating bacteria and reduced the inflammation.
It is generally safe to have dental procedures while undergoing cancer treatment. A patient just has to let the dentist know what type of treatment it is and if they are experiencing any dental issues. A lot of dentists have experience in treating cancer patients or if not, they can send you to a dental specialist.
If she has any doubts because of any suggested dental treatments, I suggest that she ask her doctor/medical team if a certain procedure is safe while doing treatment. Best of luck to her going forward. ((((HUGS))))