For what you have in mind, (I'm assuming one or two flight "lessons" of an hour or two each), gratuities would not really be expected. Typically, an instructor might receive a tip, or a gift certificate, (or a bottle of Scotch, etc.) from a student upon completion of their course after a longer term relationship, but not generally for a one-off session. In any case, I doubt anyone would be offended one way or another. I'll be curious to see if other posters have a different viewpoint on this.
By the way, I'd highly recommend scheduling for crack-of-dawn flights, like launching at 6 or 7 am. It gets hot and turbulent very early over the desert.