Dating an ex's friend?

Ok I started hanging out with my ex's friend recently strictly platonic until we ended up making out after the sixth time that we went out together. Mind you me and my ex have been apart for like four years and we have minimal contact. Now my question is my ex's friend said that dating me would be impossible since I was with her friend and that it would bother her even after all this time, yet she'll make out with me. I completly don't fully understand this so if anyone has insight please let me know. Plus I've been pretty into this woman for a while now and I just don't want to screw anything up so any advice is welcome.