How to tell someone it's time to put their dog down?

We have a relative living with us and my parents have told him nicely before that it may be time to put his dog down. The poor dog is practically completely blind, and def, and she is STINKING up the whole house SO bad. The poor dog is probably miserable. And lately she has been going to the bathroom in the house. And he doesn't want to put it down because he thinks it would be cruel. So how on earth would you tell someone that they really NEED to put their dog down? I know that this is a really touchy subject, believe me, we're going to have to put our own dog down soon as she is sick, with something that can't be cured. :( Thanks for your answers