Harris shouldn’t let any lies slide. Like every time Trump says some bullshit, she should point out first in her rebuttal that what he said was an outright lie. Biden didn’t even try calling him out on it.
Then when he is so incoherent that there isn't any lie to refute, point out his incoherence. Or ask the hosts to clarify his point, so a third party confirms his dementia riddled ramblings are not worth responding to.