That does seem concerning to come on sudden like this.
Is she still managing to eat normally? Not slower or less food?
Is her drinking and urinating habits the same?

You could top up her diet with a table spoon of rolled oats and perhaps some sunflower or pepitas. Maybe look at going onto a pellet for senior rabbits, even an alfalfa based one. Some older rabbits need this.

But I would probably have some bloodwork done before bringing in changes to pellets that may have higher calcium/protein content.

Just to add:
There are a lot of reason a bun could lose weight, especially an older one.
They can start to develop some dental issue which might drop weight and lessen ability to groom themselves.
Or they might have had a dormant infection (E.cuniculi for example) that is presenting symptoms later in life. If the renal system is not operating as it should, you’d see weightloss.
Or as you mention, cancer may debilitate.
And just general aging may have them not keep weight on as well, although that is more gradual.

Sorry to hear of her troubles Are there any other symptoms apart from the weight loss? Do her hind legs seem weaker? Is she drinking and eating more or seem more lethargic?