Can you enter Thailand on a visa exemption if you have an unused tourist visa in your passport?

is it a visa sticker or eVisa PDF?

only what your friend can do is ask to the immigration officer at passport control upon entering. but be aware the officer doesn’t have to do that. They may just say no.

if he has visa sticker in the passport, most likely the officer would see it when she/he goes through the passport pages.

in case it’s eVisa PDF format, we have reports like this. Those who had SETV eVisa PDF didn’t show it at passport control, hoping they’d use next entry. they found that their visas had been already voided when they attempted to enter with supposed-to-be-unused-SETV again later.

it totally is at discretion of the officer who would handle his entry.