If you already met her personally, then marry her is the best option, you don't need a working visa if you already marriage visa. But if you only met her online,maybe it's a trap. Pattaya is one of the red light zone in Thailand.
isn't so suspicious why his thai gf didn't asked the bank what might the problem is? Especially that money came from her bf, she should find out why, but seems she didn't do anything, if she's not a scammer.
I suggest you have to consider their advices bro. Think about it. Love is blind you know. Once she/he started asking you money, that's a red flag waving