Absolutely not wrong for feeling upset! It is 100% illegal to do what your partner and his mother have done. It’s actually fraudulent. Not only that, if everyone didn’t think for themselves with things like voting, where would our Government be? I mean, I’m not really interested in politics at all but I still want to have my say.
If you were so inclined, you could actually report him.

Its definitely illegal to have someone else vote for you. Why didn’t he organised his own postal vote, it was easy peasy and arrived with plenty of time. I’d be frustrated because this election has unfortunate results (imo; women and the environment deserve better than what we’re going to see). Did he say why he didn’t do it himself?