Ouch! Been in a similar situation very recently.
First off, not wise to tell MIL just yet, if ever. Why upset her, especially if you like her?
I'd suggest you or your partner have a word with her partner and tell him how much this has upset you. If you can settle the matter amicably, then fine, but be prepared to tell her and your own partner that you can have nothing more to do with her if behaves like this against someone whom, presumably, you like.
In our case, the comments were directed by DIL and other son's ex against me and my husband (DIL didn't like what we wore for grandson's christening). Like you, this was the latest in a line of incidents where we have supposedly upset the dear little princess. We decided enough was enough and now refuse to have anything to do with her. Sadly this means we don't see our son or grandson either, but at the end of the day the so-called adults will hopefully see that they're only hurting themselves and our grandson will begin to ask them awkward questions about his grandparents.
No good pretending there's an easy solution. We don't like this, but neither we nor our son need the stress that her constant back-stabbing causes (even though son can't see that at the moment), so it's better all round if we avoid each other.