If you get on well with her, talk to her. Honestly, make the effort to keep the relationship a good one. The glasses thing is trivial, but I'd be upset about the ignoring your wishes, too... however, context matters. She may have been annoyed by the way her son phrased it, as much as anything else. If he was imperious and patronising then as his mother he may have irritated her, so she smacked him down by indicating she wasn't his employee. An assertion of independence rather than a declaration of intent, if that makes any sense?
Talk to her. Explain that you want to work as a team to look after your DD and what he said worried you in case she intended to apply very different methods. If she has ideas they could come in really handy, and she too can learn from some newer ways of doing things - my mum is thrilled to learn ways to discipline that don't mean smacking, as when she was a young parent that was just what you did. The newer ways make far more sense to her. And she is good at reminding me that parenting is a marathon and not a sprint, and to let small things go instead of making a meal of them. My FIL isn't very hands on but adores them as well. They are very important figures in their lives.
My own MIL is awful - really, really awful. She has a horrible relationship with DH and will set out to upset the kids, deliberately, to punish him if he doesn't do as she wants. There have been numerous incidents which I won't bore you with. It's not a situation where her behaviour falls into the realms of normal interpersonal annoyance any more. So I have to admit, I feel sad when I see people advise things that will potentially harm a good relationship. Grandparents are important, when they are interested and fundamentally nice people. Adult disputes and conflicts shouldn't get in the way of a positive relationship. Please just talk to her - clear the air. It does sound like she's been wrongfooted by your DH trying to correct her childcare and she hit back. That's all. I hope, anyway, and worth investigating, no?
Most people love their kids and grandchildren and want the best for them. It's often tricky getting the balance okay, but where people are basically decent and there's goodwill on both sides I don't think allowing things to fester, or getting into a power battle, serves anyone well. She provides the best possible care for your baby, and if she isn't usually one to just act as though her own beliefs and wishes are all that matters, then this is hopefully a minor blip between mother and son and no more.