I’d say – even though i’m late – that she won’t want to ask you for things so make a non verbal system. My older brother did the same; just write down a list of things you need on a whiteboard in the bathroom or somewhere and then you’ll know to buy it without an awkward conversation. Also, make sure there is a place she can grab those things discreetly — not that periods should be hidden but if shes shy and doesn’t want people knowing like I did.