Boyfriend and his best friend

So me and my boyfriend been together for a year now and I'm very much in love with him. Weve had a rocky start, hes ex was his friend but he finally open his eyes to how much of a cow she was. But it's his best friend I'm not keen on .. were both 25 and hes best friend is 19 and female.. when I met for the first time they were play fighting and they text all the time so I said to my boyfriend it's a bit unusual so can you not text her every second of the day. So hes stopped that and stopped the play fighting. But then I keep getting messages from her about how she doesn't want to loose him blah blah. Should I be worried? Or what techniques do people use to fight off jealousy?