Recently I friended my friend's boyfriend on Facebook. Several months later they broke up and I unfriended the ex boyfriend, sending him a very nice note about my allegiance to my original friend and the need to keep that bond strong by breaking virtual friend ties with him. He was lovely about the unfriending and we parted ways very amicably.
Just yesterday another new boyfriend of a friend friended me on Facebook. I told my friend about my social media etiquette policy on friending and unfriending boyfriends and she was grateful for the update.
Lesson: it's totally okay to unfriend people's ex's.