Generally speaking, lactose overload stems from too much milk, too quickly. There isn't really any such thing as foremilk and hindmilk - they're very confusing terms. The only thing that really changes through the feed is that the percentage amount of fat increases. Often mothers are told that the 'foremilk' is watery and sugary. Well, breast milk is mostly water, yes, and there is more fat at tthe end of the feed, but there is no magical point when your milk changes from one kind to another, and there is no change in the amount of lactose in the milk.

The point of all this is - that it should be fine.

If she's having issues with oversupply and/or forceful letdown, then it might be helpful to pace the feed if you're using a bottle, so it comes nice and slow. How long has it been in the freezer? It might keep for some time yet.