connect to wifi w/ a static ip and no gateway address?

Customer is going to be onsite in another state. I need to remote into his computer and connect to his plc.

I know that I could do this if his laptop was connected to the plc via lan no problem. Either change the metric so the laptop gives internet priority to the hot spot, or make the lan static and remove the gateway.

I have however, not tried to do this w/ the wireless adapter. i.e. can he still connect to a wireless router if i give the wireless adapter on his laptop a static ip w/o a gateway?

You can still connect to a wireless AP w/ a static IP and no gateway yes.
You will not be able to route anywhere unless you define a static route in this case however, or have another connection w/ a route defined.

If, You need to connect to a single network and use different gateway address for things, you can define a primary gateway in the connection and add static routes for other things. I've done this where I had a trunked port for my PC and needed to direct differnt traffic to specific VLANs.