Trouble with & ex girlfreind.?

Me and my knew boyfreind have only been together a short while but are very happy. the only thing bothering us is his ex girlfreind. she went out with him causuall dating for around 6 months but for around 2 and a hlaf of these months he has been trying to break up with her by trying i meen he break's up with her but she continuously still refers to him as her boyfreind and continuously texts him how they should be together and she cant live without him which he replys to her tryign to explain why they cnat be together and he dosent wnat a relationship etc... me and him were good freinds while he was strugerling with this problem and would look to me for advice during this time we realised our feelings for eachother and started a relationship. to make things more complicated his ex is in the same calss as him at college everyday mon-fri, and she used to be a freind of mine when we were young at the age of around 5 this makes it seem complicated as she dosent know about the relationship but suspects it and calls me crying saying hes the love of her life and as me and her are freinds i should not try and be with him which makes me feel guilty. how do i know if he is using me to get to her which i only slightly suspect as we havent told her off the relationship and he says he has not felt like this before. and how should i aproach lettign her know about us being together. she also has a very bad anger problem meaning she cna be very violent and i am worried she may try to start a big fight with me. plese help?!