Its his part of job in his own country. So he will not be paid in Thailand but might be given extra allowances along with his normal salary

Many tour companies from foreign countries send their staff as a tour leader with the group who act more as a superviser rather than a guide as there will be a seperate Thai guide. I used to do the same. I don't think there is a need of work permit or any special visa for that since a tour leader is just a part of tour group and return with the group.