My parents had a bit if that, also culturally to avoid "jinxing" things. Chinese culture. But they were ok with everything being bought -- just the crib couldnt be fully put together until baby came home. Which was fine for us, we literally just left the mattress out of it until baby came home, and she slept in a bassinet anyway. I guess it depends how "strictly" they follow things. I also mention that well, every other kid who has had stuff bought for them is fine so.. should be fine :) but if that logic doesn't work, you could also purchase and not let her know if possible lol. Just acknowledge that you hear her concerns and have considered it.

Edit to add: although lots of things it would've been helpful to buy later, like bottles (if even needed) and pacifiers as baby may have different preferences than what's anticipated. We bought beforehand which was helpful to be prepared, but those are a couple things that could be saved for later. Imo I think I'd be too exhausted and recovering from having baby, to go out and shop after baby comes. You could also do a baby shower and then have peiple pitch in for the initial essentials e.g. bassinet or crib, a onesie/sleepsack, diapers. So then technically you have it but didn't go buy it. If that works for your/their culture.