Angel was diagnosed in April 2016 with lymphoma and breezed through chemo (CHOP) with absolutely no perceivable side effects - she stayed in first remission for 18 months. She's almost 15 now.
That said, ironically, she is not doing well at all right now (not sure how much is disease and how much is neutropenic infection)... so I totally get your concern.
In my opinion, it is a gift to be able to help them pass, and never the wrong time if the decision is made with love.
Many for whatever you decide to do for your sweet girl.