First, I think he should start off by calculating the total cost for his hotel stay. It is easy to get an estimate for the accommodation. Then, he could think about the budget for food (each day). Followed by entertainment and other activities. I think the best way is for you to discuss with him what he wants to do when he's in Thailand (plan out the itinerary). After that, you should be able to come up with a rough number of the total expenses. That's what I do when I travel.