Anyone know of or have any insight with chemo causing dementia and incontinence?

Wendy, Google "chemo brain" to help give you a better understanding of what is going on with your parent. The American Cancer Society has an excellent article.

Chemo brain is a very normal side effect of having anti-cancer meds, be it chemo or in pill form. I know, I have the t-shirt.

To be fair, your oncologist knows about cancer, not dementia. I'm sure that technically the chemo doesn't CAUSE dementia, but it pushed the timetable forward. Any kind of stress in the elderly, be it infections, surgery or chemo can trigger a cognitive decline. She may recover somewhat when chemo ends, but she may not.