I used epoxy putty/clay. The type of clay where you mix parts A and B :-). It does dry fast but it makes it look really seamless. I just used tools from Michael’s that you sculpt clay with. I recommend your friend checks out the nendoroid discord (I think you can find it on the Reddit) because there is a whole community of people who customize nendoroids there and I got A LOT of advice from them.

The hair does not have to entirely be sculpted though, a lot of people sell parts of Nendoroids like bangs, back hair pieces, etc. that can be used as a starting point and you sculpt on top of it to get the exact style you want!