I think my DM is consistently faking dice rolls for saves against a specific spell; how do I call my DM out?

I’ve been watching one of our players repeatedly cast toll the dead (Xanathar’s Guide to Everything, p. 169), across seven sessions, and a dozen different combat encounters, and the DM has never once allowed her to do any damage with the cantrip. She has a spell save DC of 16, yet our DM always “mysteriously” rolls the saving throw.

Obviously, “just quit the game”, “that group is not for you”, are the answers most folks will immediately suggest, but I’m not the one playing a warlock and I feel like telling her to quit would be awfully rude of me. She’s a really quiet and shy person, and I can’t help feeling like someone needs to stand up and defend her. Last session she looked like she was on the verge of tears.

Anyone have a creative method of calling your DM out for being a dice cheat in front of the entire group? I’m really disgusted by his behavior and I’m guessing that statistically speaking the permutation is so large by this point that his monsters have won the powerball ten times over.