There are 2 proofs of funds. First is what is required to get the visa. That will vary based on the embassy used. You need to show whatever they request in the bank account as proof to get the visa itself. Second is what is required to enter Thailand. Technically anyone entering as a tourist could be asked to show 20,000 baht in cash. Not common if you have an actual tourist visa, but they can still ask if they want. I think people from neighboring countries have traditionally been treated a little differently when it comes to being able to enter multiple times, and they are not scrutinized as strictly as westerners and other foreigners. But there's still a concern over the person potentially coming into Thailand to work illegally if they are spending too much time in Thailand. Basically she'll be fine entering Thailand multiple times, probably not even needing an METV (but it wouldn't hurt), until she can't anymore. The only way to be sure about coming as much as she wants is to have a long-term visa, but that's not always feasible for some people based on age, and other factors.
