i express milk at work and so friday's milk goes in the freezer and I take the oldest milk in the freezer out for monday. I believe it is best to allow it to thaw in the fridge. So I take what I think I need all out at the same time on sunday evening and leave it in the fridge, then as Isabella's nanny needs it she thaws it more by putting it in warm water.
You can definitely only use thawed milk for 24 hrs.
I would tell your mom how much roughly she needs per day, take that out to thaw each evening. If she runs out, she can get more in the freezer and do a quick thaw in hot water. It thaws pretty fast that way. Since you have lots, you won't mind as much if a little had to get thrown out. I on the other hand have found it hard to get enough pumping during the day and so the milk is like gold and I do everything possible not to waste a drop. I wish I had stocked up more when i was home on maternity leave but hindsight is 20/20 vision.