How would you respond to this type of dishonesty?

I am being approached for advice and I feel like I’m out of my depth – so I figured I would ask the tidepool.

My friend (S) has been fighting with her boyfriend (B). B called his mum (M) during the argument and portrayed S in a very bad light, neglecting to mention his own mistakes and highlighting hers.

As a consequence, M now does not approve of S (though she was quite a fan before). She has advised her son to break up with S. B does not want to do that, but S is now upset and embarrassed.

Should she ignore all the drama, or is there an adult way of dealing with it directly?