Just my satang's worth.
I wouldn't even look at this as employment. I think RichardSt's wife just wants to mix with some Thai people, eat some Thai food and keep herself busy. It just so happens she gets paid for that. It's like Phetchy and Dave meeting up for a bite to eat and the restaurant paying them for their time.
If she is serving it should help her confidence in speaking English in a real situation. And if it stays at just a couple of nights a week then it shouldn't affect her studies.
If you stop her doing it I think you might have a problem because she will not understand the rigidity of the U.K. law in this respect - there is a minimum wage in Thailand.
Even if the restaurant is raided by immigration - and what are the chances of that - she is in the clear.
And McDonalds can afford to pay higher salaries by not actually serving real food, and having the financial clout to extort their suppliers. There is no comparison with the cost of imported hor la pa and makua.