Re: would you intervene?
I think it is very reasonable and logical for the mother-in-law to ask these things. If one is going to make an accusation as serious and slanderous (in the event that it is not true) as the one your friend is making about this gentleman, one must be absolutely certain that what they are saying is true. This would require considering the exact source of the information.
Also, as I stated earlier regarding proof, what your friend is alleging is of a very personal and private nature, so how does she know for a fact that it is true? Hearing from someone or having someone tell her these things is not proof. Overall, I think that your friend should not get involved.