Could you ask how she'd like to spend the rest of her holiday? She might want/need some time alone if she does, then let her. You can still go and do what you planned. If it's nearby just tell her to text if she changes her mind.
Can you include her in the smaller decisions that don't require her to really do anything but shows you haven't forgotten about her?
*MiL we can't decide where to go/what to cook for dinner, what do you fancy?
If she doesn't want dinner, that's ok. I'd probably say there's some stuff in the fridge or some leftovers if you get hungry later and leave it at that.
*MiL we're at the supermarket, do you want us to pick up some more of that ice cream/snack/juice you like?
No she doesn't want it. That's ok, just grab the one you like.
*MiL if you don't feel like a picnic on the beach, I'll leave you some (insert picnic foods) in the fridge for you.
If she doesn't eat it, somebody will polish it off on a midnight raid of the fridge.
As another poster said, grief is not linear. Sometimes it comes back and kicks you in the face again. Perhaps she's been able to put on a brave face for short durations but can't do it 24/7. You're seeing the full picture of her grief. That's probably hard for her too. Bless her ❤