Is she being immature and unreasonable?

I'd been friends with this chic since grade 4 but after finishing primary we only saw each other about 4times a year for the next 5yrs. I meant her (then) bf at the time on one such visit, they had been together about a month. After that I had not seen nor spoken to him in 2yrs. Out of the blue he contacted me (18months after they split) and we started dating. My friend got very jealous and upset. She doesn't talk to me anymore and gas deleted me from all her Internet accounts. Apart from that she slandered me infront of her mates (whom some are friends with my mates), it caused a lot of drama. What are peoples opinions on this matter???