Sorry, but that’s crap. Questioning the credibility of a liar is not automatically fallacious reasoning or an ad hominem. Attacking their character instead of arguing against their points is an ad hominem fallacy. Pointing out the consistency of lies from a single source and then extrapolating out to question the validity of future statements of fact is rational, logical, and reasonable. It’s perfectly valid to label a liar when they repeatedly tell lies, as long as you can support the label by proving they are lying.