Is it bad to like my friends OLD crush?

Me and my best friend agreed we wouldn't like any of our old crushes or current ones but her old crush she liked for two years :// but all through that time I REALLY liked him but this year it's gone away as I liked someone else but when I was rejected by that guy and my best friend had ditched me but then came back I started talking to the old guy again and starting getting feelings for him again! If my best friend'd old crush asked me out would it be bad for me to say yes? I feel really bad and I'm sorry if this question was worded strangely, I'm not always good with getting words out!!