Nope.
As marko said, you can have a creature ready to trade bleak immortality for a few bright years, but still, as YOU deem preferable to live forever, you decide to deny it to him, based on your choice. Then again, just as you could say to a paralitic "I choose for you to live forever in this state, instead of being whole and dying in 10 years". Their life, their choice.
Franckly, to me, the biggest problem here is that you see immortality as eminently desirable, and can't conceive that there might be trade-offs.
And sure, this choice might be affected by their affection for the magus, but maybe not in the way you think. If clara dies, wirth will die, although he plans to live forever... With her. But life without her is nothing to him. And there's no familiar bond in it
In fact, he would see such a bond like a blessing if the death of his wife would mean death for him. Don't you think some familiars might think along similar lines? Such degree of commitment can also explain why it is so difficult to find one, instead of taking the magical cat next door.
Moreso, to think that the familiar would do this to keep the magus affection means, IMO, that it is not a suitable familiar, since true friends don't feel the need to do anything to keep the friendship of their companions. There need to be real trust and friendship between a magus and his would-be familiar, and this precludes such feelings.