If you’re working with Google Web Fonts there’s something you should watch out for. I was working with one of my favorite fonts Open Sans and the 300 weight version looked perfect on my computer while too thin on my friend’s.
I realized that I had the Open Sans font installed on my Mac, so disabling the font (or deleting it) helped me look at the website the same way my friend was viewing it and yes, I had to grab the 400 weight version to achieve the results I’ve seen before on my screen.
What OS were they on? I know it's not free like Google Fonts, but TypeKit seems to be a much better alternative. They appear to have taken care of reducing the differences seen on browser/OS combinations.
Friends? Doesn't really matter since Open Sans is not pre-installed on any OS as far as I'm aware, I just installed it on mine for design purposes so I was actually the "buggy" one, not them :) Thanks for your comment!