I know you are a nice person missprim and not intending any harm whatsoever, but like Lottie I am ( usually, not always) in favour of letting old lives tick themselves away unless the animal is in pain. Old things do wind down, sometimes they gain weight, sometimes they lose weight, they go deaf, blind, become incontinent and many other things which distress people who see them. That does not necessarily mean from their perspective they have no quality of life. My old wolfhound was blind and deaf and very unsteady on his legs, suffered from vestibular attacks and generally looked awful, but I knew him and knew him to be happy just very decrepit. He died peacefully and happily on his sofa in the comfort of his own home. I know some people would have considered a blind and largely deaf dog to be suffering, I personally don't view it that way as it's a gradual process and one which they become gradually acclimatised to.
If you would like to help your friend, offer to take the old dog on a very short walk, she may have simply gained weight because she's lacking exercise and take it from there, but try to be gentle about any suggestion that the dog has reached it's end, as you might finds it's human companion thinks differently.