Using a gfci to feed a pool sub panel is not a good design. I would not do it, just wouldn't be prudent. Even though a 2 pole 50A gfci is cheaper and more available then a 2 pole 20A gfci.
We've been using Hayward as Dennis said Siemens works well with them..
I still do not feed an entire sub panel with a gfci.
Haywards are getting hard to get because so many people are using all the govt covid money to build pools and buy boats. Haywards are the best choice. We're wiring one pool after the other. In Indy they all get heaters. Ours get pool covers, salt, LV LED lights, perimeter pumps too.
Is your friend powering the pump with 240V or 120V?
We're using Jandy pumps because we cant get Hayward right now. They're a poor design IMO or at least more complicated as they use a controller that is mounted on the wall near them.