DHCP, separate DNS

I have a pihole and it blocks anything Facebook. I have no FB account, but my wife does. I want my machines to go through pihole and hers to quad9.
I could hardcode hers with a fixed DNS, or mine. But i was wondering if I could automate all of it. I have a HEX gr2, that NATs, does dhcp, does local name resolving and finally forwards dns requests to the pihole. DHCP now points all clients to the HEX fod DNS, but I want my wifes machines to be handed 9.9.9.9 instead.

Thanks for any ideas