How to compare wages across countries

So, there's a thread over in r/todayilearned, which has trickled up to r/all, about Beyonce's clothing line being made in sweatshops where women are paid 5 pounds a day. The big question in my mind is, how do we make an accurate judgment about cases like this and what a "fair" wage would be. In that thread, you have people arguing that what the workers are being paid is twice the minimum wage in Sri Lanka. Others counter that the company COULD pay them more and still make money, so therefore they should. So when comparing wages from one country to another, of course we have to take into account currency exchange rates, but also the differing levels of buying power (which comes up in US minimum wage discussions as well - cost of living matters). So my questions for the economists of Reddit are:

1. Are there any other factors that should be taken into account when making a moral judgment about things like this?

2. Is there a relatively neutral source of information that I could use to make these comparisons myself? I know how to find currency exchange rates, but I'm not so sure about comparing purchasing power.