Why do you care about how they vote? It literally doesn't matter.
I agree that voting doesn't matter in terms of making a material difference in the social hierarchy because the US is not a democracy. However, who a person votes for (or whether they vote) is an indication of their values, which are in turn an indication of their future behavior, and anyone who votes for the regime that is currently conducting a fucking genocide demonstrates either their ignorance (which it seems OP's friend cannot claim as a so-called socialist) or a fundamental lack of solidarity with those who are being dehumanized, tortured, and murdered, the victims of that genocide. It demonstrates a fundamental lack of solidarity with all oppressed peoples of the world. Just because voting doesn't matter with respect to who wins the election, it sure as hell matters on an interpersonal level.
Find it pretty funny how electoralist brainworms have infected even socialist Americans to the point that they'd think convincing someone not to vote is a useful course of action.
It has next to nothing to do with electoralism as it is only tangentially related to the election itself. Again, it is about a person's position relative to class struggle. If a member of the ruling class were murdering your family and your friend decided to throw in their support to that murderer with the rationalization that that murderer might benefit your friend more than some other murderer, it doesn't matter how inconsequential their support is - it still tells you a great deal about your friend's priorities and interests which don't actually include your dying family or other oppressed people, only themselves.
Or go vote to better maintain a friendship. Or lie about it. Again it simply doesn't matter.
Everything else aside, this is still terrible advice. Lie to your friend about your principles regarding genocide to "better maintain a friendship"? Gross.
Again it simply doesn't matter.
Socialism is not nihilism.
Anyone who uses the term "tankie" unironically to disparage other leftists is a lib. Anyone who uses the term ironically to make fun of those who don't is a leftist. But you're right, conservatives don't know what it means, they also don't know that they are libs as well.