Honestly, if anyone ever tells me they won't be my friend if I'm friends with someone they don't like, I will not choose them. It might not even be my preference. However, that would be their choice of revenge over friendship. I have had this happen in person a couple times in my life and either that person would not talk to me anymore or they would learn to get over it and talk to me again. No matter how guilty that person makes you feel, you do not need to make that choice for them so they can take the blame off themselves.