If Side A accuses Side B of lying – especially when the accusations are specified, the best thing that Side B can do is show that they’re not lying.
I know that this isn’t usually how the onus of proof works. It’s not incumbent upon the accused to prove their innocence, but if the truth is on your side, why not use it? What better way to make the accuser look utterly foolish?
If Side B won’t do that, then they look like liars. If they can’t do it, then they are liars. If all they can do is complain about how unsporting it is that someone said they lied, it really doesn’t look good.