My friend is thinking about coming to South-East Asia with me, but he has drug charges on his criminal record. We'll be landing in Bangkok, Thailand. Is that a problem with tourist visa's (VOA)?
For starters your friend does not get a VOA (Visa on Arrival), rather he enters under the Visa Waiver program as a Canadian citizen. Which, as you likely know, gives him a 30 day entry stamp each time he comes.
His criminal record in Canada is not available to immigration officials in Thailand, unless they have requested it from the Canadian government, which is highly unlikely since he has never been in Thailand before.
If he applies for a 60 day Tourist Visa in advance, then the Thailand Embassy in Canada may request and consider his criminal record. The same would apply to any other country which requires him to apply in advance for a tourist visa.
For VOAs in neighboring countries (Cambodia, Laos), the visa is issued based supplied information on the application form and any data in that country's immigration system from previous visits. The immigration officials do not have access to nor the time to comb through other country's criminal records.