There is nothing to stop you wearing whatever you like. Your friend knows about your cross-dressing , so it would probably be no great surprise if you turned up to help her with the chores in a housemaid's black dress and white cap and apron, or a charlady's crossover pinny and headsquare.
The question you should be asking is - Are you really concerned to help her? If you are, then you shouldn't have any problem asking if she needs a hand with the housework. Once that is established, you can ask about the dress code.
However, if what you are really looking for is a chance to put on your glad rags and pose, you should be considering whether you are exploiting her for your own purposes. Only you know which it is.
There is a third possibility too - that this is a fantasy of yours, which you are airing on Gransnet to get reactions from (presumably shockable) older ladies. Consider too whether it could be exploiting Gransnet posters - a kind of virtual indecent exposure?