To tell MIL to let the poor dog go? (Sad and possibly triggering to dog owners)

[ohyesohyessyyesyes]

So I went to my MIL (for want of a better word)’s house today for the first time in ages as I am no longer married to her son. Jesus, her poor dog ☹️
She loves him dearly, no question.
He’s 18 years old, can’t stand up properly, can’t walk more than a few wobbly steps, can’t see well, barely moves, doesn’t really eat.
He was just lying there with some flies on him.
She told me that he shits himself in the night and as he doesn’t eat properly, it is diarrhea so every morning she has to carry him down to the shower and clean him up.
I wanted to tell her to just let the poor thing go but stopped myself as I didn’t want to upset her.
But I can’t help feeling sorry for the dog. That isn’t a life. It’s somehow cruel for her to be keeping him alive just because she hasn’t come to terms with letting him die. Maybe I should say something after all? She is super sensitive by the way and would certainly be upset by the suggestions.

Yanbu to tell her it’s ok to have him put to sleep.
When our dog went like this, at first we felt it wrong to pts just because she was doubly incontinent, as a carer im used to cleaning up people all the time. But he’s not human and all he knows now is shame. What joy does he have in life? It’s not as if he can sit and read, watch tv, do crafts, listen to music like a person can.