Ethics of dissertation / thesis proofreading OK. Short version, I'm stuck on my PhD because of coronavirus. My university is giving me an extension to my thesis deadline, but I am going to run out of funding pretty soon. 

I'm looking at marketable skills I can use to generate a bit of income to bridge the gap, and one of these is scientific writing. I know that there are plenty of people out there who have English as a second language, or who don't express themselves fluently and I think they'd be interested in proofreading / light copy editing. I absolutely don't want to become an essay mill, and I don't want to help people to cheat.

Can anyone advise me on the ethics of this? Where do the boundaries lie?

Thanks!
Proofreading and even light editing for proper language usage and understandability should not be an issue at all. You are not contributing to the actual work, just to its explanation. Advisors sometimes do this sort of thing. 

I'll assume of course that the theses you read aren't fundamentally about the language itself. But proofreading a math paper, say, helping the author to find the right English or German or whatever words and phrases for expressibility is ok as long as it doesn't change the content or the ideas. I'm assuming, also, that the advice you would give is non-technical. 

However, the best way to do this is to make suggestions to the author about places in the text that need improvement, rather than doing a markup. This aids the author in developing professional language skills and helps assure that you don't cross boundaries. 

If you are in the same field then it is easy to go too far, so you need to guard against that. 



I should note that some people take a very strict view about such things and would forbid it.