I think she qualifies, but because there are very much people cheating for this visa, she might be rejected by some embassies.
There is no income needed, but it might be the best idea to work online first for a short time to make a better chance, but with a good motivation it might work with some embassies as startup.
I would in her case not make a problem of remote working for sometime, she can even ask if that is allowed.
Thai authorities should clearly tell how they see remote working, but they don't.
Before DTV I asked Thai embassy if I could work remote during my stay in Thailand and they said it was no problem.
On the DTV workcation now is clearly stated working prohibited, so the conclusion might be they do not see the remote work you do as working in Thailand.
On the other hand, when I am in my own house in Thailand, using a VPN, who can see I am working?
workcation for 6 weeks 5 years long is a much better reason for a 5 year visa, than doing Muay Thai training for six months and then stop the training. Of course technically that is illegal use of the visa, but it happens a lot. When you get a visa for 5 years, and only use it 6 weeks each year for workcation, it seems completely according the rules to me, but as always, embassy will decide about that.
I do not agree with you, it is not remote work. When what you say was true, that would mean you could do any work you want to do inside Thailand, would be allowed without work permit as long as you are paid by a company outside. That just is not true.
HR saying anything to you is not enough, you need a letter from the company you are allowed to work remote, preferable with Thailand mentioned or at any location.
When remote work is not mentioned in your contract, the embassy will ask you normally for that letter.