There have been instances of people being denied entry on the MetV visa because the immigration official has suspected them of living in Thailand illegally.
So it is important to take note that a Multiple Entry visa does not give you impunity when facing scrutiny.
Things that will raise suspicion may be same day border bounces, consistently crossing at the same border and spending the vast majority of your "travel period" in Thailand.
They have the right to ask you for a flight out of Thailand and to see 20,000thb in cash everytime you enter the country (I believe there is no difference in criteria to the SETV) and ultimately they have the right to deny your entrance to the Kingdom and once they do that it is very rare their decision is reversed.
So, if you plan to come to Thailand each year for
****
months I'd be interested to see how long you can do this for.
It's all about the documentation. If she has things in Thailand like a mortgage, assets etc. things she couldn't just bail on if she wanted to emigrate then it shouldn't be an issue.
Visa cost is about 8,000thb all things considered.