New to the iVillage board.
First of all I think it is not up to you to ask your MIL of only 4 mos. to stop being friends w/ your DH's ex--if it bothers him, he should be the one to say it.
I don't think it's your place to "tell" the exwife to do anything.
I would like to say first and foremost NO, to answer your question, you should not be saying anything to MIL that is completely DHs place to deal with his mom how he sees fit.
I scanned the thread and generally agree with what has already been said.
Just like you talked to your mom, your DH needs to talk to his mom.