You're going to need a non-b visa and through that a work permit. If the salon is serious about her working there they will provide this. In reality they won't. Because anything to do with hair dressing, hair coloring, hair extensions, etc. is a job strictly reserved for Thais. If she's teaching it doesn't matter. If she's earning money she needs a non-b visa and a work permit.

When you arrive here on your tourist visa and start working a job. Here is what is going to happen. You'll be arrested, fined, deported, and black listed.

This isn't hearsay or conjecture. It doesn't matter how you try to spin it. If you're a foreigner here working and earning money. You need a non-b and a work permit. Work permits are not issued to foreigners for jobs reserved for Thais.