I'm not surprised you're furious - it is quite wrong of her to ask ds to lie. How old is he btw?
I think when your dh talks to her it would be highly inflammatory to use the exact phrase you suggest, but something definitely does need to be said. I've had awful problems with my XH, and know from bitter experience that throwing accusations of bad parenting just isn't constructive for anyone. I would advise your DH to discuss the specific matter in hand and make it clear to her that DS is upset that he has been asked to lie - i.e. emphasize the effect on DS rather than criticise her directly.
Sorry if I'm just stating the obvious - I know how hard it is so hugs to you and hope she has the decency to realise she has done wrong X