That's a lovely idea, Bumperlicious.
Chemo does not always mean hair loss, it depends on the drugs being used. But it very frequently does, especially for those cancers that are hormone related - certainly breast ones, and I imagine ovarian as well. But it may be as well to check on the drug regime. Your aunt will know, as she will NOT be offered false hope on the hair front. If it's likely to go, they'll tell her.
A hat that lets the head breathe, but gives good cover so the baldness does not show through, would be a great idea.
I have had BC and, in my experience, she'd be wanting something slouchy that would follow (or at least cover) the hairline. (Something that just sits on the top - like a baseball cap, for example - is not so good, as it exposes the bald head lower down. Deeper colours are more flattering than pale ones, especially if she has dark natural colouring.
Feel free to ignore, of course, but this is my experience (I now have a full head of hair again . )
HTH