Is she driving me away from my fiance on purpose?

I have been with my fiance for 2 years and engaged for nearly a year. he is in the army and is based down south and i live 400 miles away. The long distance i am used to and i love him to bits. its his friend Amanda i cant stand. she lives just a few minutes away from him and she has bipolar and whenever she is down she talks to him, at first i was ok with this. However, since then my mates sent me a screen print of his emails with naked photos of her on the email. Even though i felt sick at the thought i forgave him and didnt tell him i knew (this was before we got engaged). Last night though made me think, she has been going out with one of his friends who just so happens to have the same personality as my fiance but looks different. he dumped her and my fiance got so pissed off he had to have 3 people stop him going round there and punching this other guys lights out. I just think that he would run and save her at any time but i get left on the sideline. Should i confront him about the photos and ask him where his priorities lie? or do i just leave it and see if anything else goes on first so that i have full confirmation first. I really love him and dont want to lose him but Amanda is driving me away. she even text him when he was on leave and told she loved him, he replied saying what?!? sarah saw that msg. she responded with sorry, i meant as a friend. i trust him but i dont trust her and shes invited herself to the wedding.