He told me that was one of the things he was going to do, but I don’t think he actually went scuba diving
I know that to rent scuba equipment you have to at least have an "open water" diving certification, even in Thailand. I know someone who just got their SSI (Scuba Schools International) in Thailand. You have to have the SSI or a PADI (Professional Association of Diving Instructors) certification to even rent diving equipment anywhere. I myself am PADI certified.
I highly doubt that he would spend the money and time to get certified.
To combine a liveaboard trip with a scuba diving course, a minimum of 3-4 days onboard is recommend as the Open Water course takes around 3 - 3.5 days to complete.
Another option is to complete your (Open Water) diving course from shore. One of the cheapest options available is to get your diving licence in Koh Tao, Thailand. Prices for a Open Water courses varies between USDS 200 - USD 300. Sometimes, free accommodation during the course is included.
Quotes taken from: PADI vs SSI certification for scuba diving: Which one to choose? | Scuba Diving Maps