Voter fraud, in the context of the 2020 US election, is the concern that hostile foreign powers or domestic actors will flood the system with thousands of fraudulent votes. This is a very real concern and has been raised multiple times not only by Donald trump, but also serious figures like Bill Barr, see https://www.youtube.com/watch?v=mAsOtb1_FLg, who has access to the relevant facts.
This concern, which could steal the 2020 election, is entirely different from a single person speculating, probably in jest, he might vote on behalf of his spouse who no doubt would vote the same as him anyway. In one case ballots are being made up by the thousands, and in the other an already printed ballot is simply filled out. One case is a very minor issue which could hypothetically result in a small fine, the other is felony and a multi-year stint in federal prison.
Equating and both-siding the two situations serves nobody but Joe Biden and the people who are interested in stealing the 2020 US election.