Can I book flight one way with cash and another with Qantas points?

I’m trying to book flights for my sister to come for the UK to visit me. Looks like I could get classic rewards for the return journey but it’s not available for incoming flight.

Am I able to book half and half in one booking (cash one flight, points the other) in one transaction… doesn’t seem possible when I try it on the app

Or, could I book her out here on a cheaper flight for cash with another airline and then have her go home using a flight I pay with my points with Qantas?

Just wondering the best way to go about it…any advice appreciated!