Tamales with rendered turkey fat and clarified butter - No pork?
I recommend clarifying the butter first, and then using it for your tamales. Butter is about 16-18% water, which could significantly affect the fat content of the dish. If you do that, it should work fine from a mechanical perspective; the melting points and viscosity are similar enough. I think you may run the risk of being slightly distracted by the flavor. Maybe that's a good thing? I make butter/flour tortillas all the time and those are freaking delicious.

The safer bet, of course, would be to try and find some chicken fat. If you can't, I'd definitely use the clarified butter though. Who knows, it might be your new go-to secret ingredient for tamales!

::Columbo voice:: Just one more thing.
If this person is avoiding pork because she's eating kosher, using butter with meat is likely a no no as well!

Good luck!