Cafe offering to pay less to foreigners

There is a small cafe that my friend and I often visit to chill and study, and we thought it would be a fun idea to get a part-time job there. However, when I interviewed, they told me they would pay me minimum wage, but when my friend interviewed, they offered to pay her more (it's not that much of a difference, but 5-10% more than minimum wage). My friend was upset when she found out and said she wouldn't work there and even boycotted the cafe, which was nice of her. But what worries me is that this kind of mistreatment could happen elsewhere even if I find a job in Korea, and I could be paid less than my peers just because I'm a foreigner. Is it common in Korea to pay foreigners less? Has anyone experienced this?