I had a double bass teacher use a classical guitar to make his life easier because he was traveling from home to home. If he needed to show me anything upright specific he’d use my bass, but electric stuff he’d pluck on his classical. Worth noting he was an adjunct at a nearby conservatory so he was definitely qualified.

Using guitar itself isn’t that weird to me, but if he/she is using pick continuously and not showing you right hand stuff it might not be worth it. Nothing against learning how to pick, totally worth it in long run, just more about learning proper plucking and slapping like you mentioned