I really think those of us who’ve never menstruated could still afford to be aware enough to be helpful. It’s not hard to find the feminine hygiene section of a grocery store. Most people who menstruate are going to pick out the product they want, but if she really did just start that minute, pick some pads somewhere around the middle size of the product line, unscented. That’ll probably work, even if she has never used them before.
If you imagine the checkout person is thinking you are buying them for yourself, go ahead and let them. Doing that work could get boring, right?
Half the ancestors to whom we owe our existence menstruated. It’s not like we don’t have anything at all to do with the process.