My client has excessive foot supination (both feet) during a squat. What are good exercises/stretches to help?

Did you assess what’s happening at the joints above?

I’d start by looking at calf and peroneal flexibility. Have client do a static calf stretch (traditional, one leg extended back long with knee straight but not locked, foot flat, for gastrocnemius). Make sure both hips are pointing straight forward, both knees are straight forward, and both feet are straight forward and back. Correct any of those issues and see what how that feels with client – could be tight with just those corrections. Next step = ensure that the big toe on the back foot is pressed into the floor so client can feel bodyweight on it. Next progression = same stretch, but in addition to big toe down, lift pinky toe up.