This is a tricky one, isn't it? So many individual factors play into it.
One is: do you have the impression that MIL told you this hoping, or even intending, that you would drop a hint to cousin as a result? I can totally see that you don't want to take it on yourself to act as the family busybody; but if MIL is counting on you to put in a tactful word that would produce a charming thank-you to which MIL could beam fondly and say 'it's nothing dear, I'm just so happy to be able to... etc', so that everything would be sweet again, then perhaps you should.
My first instinct would be to sit on my hands, but that's because my in father's family nobody ever remonstrated directly with anyone else when they were upset about their behaviour, but everyone made it their business to bustle around taking relatives aside and saying 'I think you should know that MIL is very upset about your doing / not doing X' - sometimes when they didn't even know for sure that MIL was upset, only that they figured she would be. My father hated this and threw himself gratefully into the bosom of my mother's family, Quakers who won't hesitate to tell you exactly where your behaviour is faulty, in a helpful spirit. We like it, even though it can bruise, because we know exactly where we stand with each other. But to people who don't operate in the same way, and who would stew for decades and even cut people out of their wills without ever telling them how much upset their behaviour had caused, one of our mild just-setting-you-straight remarks can come across as a crushing condemnation.