Its how long is a piece of string, depending how well she actually knows her dad and how difficult he is to buy for.
How often does she see her dad, if you live a long way apart?
My 12 year old DD chooses something herself for DH - but she lives with him obviously! Its usually something edible tbh - she knows his favourite treat snacks because she sees him every single day and spends almost every evening in the same room as him, plus he hardly ever enters a shop (really, a few times a year) so he never buys himself those little treats, and they actually are a lovely thought!
She most certainly couldn't choose for my father (her grandfather) because she doesn't see him very often, living a long way away. TBH it is impossible to buy for my parents anyway as they have plenty of money and buy themselves whatever without a second thought - there is nothing that isn't very expensive that they've been hankering after and not just bought for themselves, and it's been that way for as long as I can remember.
If your DSD only sees her dad in the school holidays and he tends to buy himself (or you buy him) all the little bits and bobs she might realistically cotton on that he might like which might be within her budget I couldn't blame her for never getting the hang of buying for her dad.
However if she sees him every weekend and they actually hang about together (rather than rushing about doing activities/ visiting relatives or ignoring one another in companionable silence on gadgets) so really know one another, and he does have things he'd like which he doesn't just constantly buy for himself, then 12 is fully old enough to choose for herself.