Sounds like they only will arrive with the 30 day free exempt Visa. if they stay longer they will need to apply and pay for an extension , can do this when in Thailand. or if not pay fine when leaving .( even for just 2 days) - Maybe do a short trip to Cambodia ( Phnom Pehn or Siem Reap/Ankor Wat) for a few days, this will give them another 30 days on return if fly or 15 days overland. also if you have a return ticket to/from the USA there should be no problem.
, yes it happened to me about 16 months ago with a lady IO. - I travel to Thailand 4 times a year for
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weeks at a time , on 30 day visa exempt. She questioned me why I come so often , do I have a wife or gf, what I do, where I staying etc ? And next time I need a proper visa. As I was coming back in 10 weeks I did a lot of research , called the Thai embassy in Sydney, went to a few visa shops in Pattaya, all gave different answers, , I just went with what the Thai Syd embassy told me, "there is no set rule on the amount of times flying in and out on the 30 day exempt visa, it's up to the discretion of the Immigration Officer on arrival.