If a person cast an illegal vote for Biden, they have entered into election fraud with Biden, and Biden is just as culpable, that would be guilty, as the person that cast the illegal vote. That’s RICO.
Biden can be charged with a crime and none of his votes would be valid.
He would also be charged with voter fraud.
Thats pretty much an insurmountable burden of proof to meet without a huge smoking gun to connect Biden directly or indirectly through an enterprise. In my experience, huge smoking guns do not come to light in discovery very often at all (happens very rarely but it sure is nice when it does), and the more money the other side has just further reduces that chance.