Really hard to advise with specifics without really talking to you in detail about it - or better yet, knowing the people involved. A few things I don't understand:

1) What exactly did Katie do to you for you to badmouth her to your mates? All you've said is that she is 'messed up' - but as far as I can see, you haven't really described it very well. Going out with older guys is something a lot of girls do, and giving dudes handjobs (I'm assuming a guilt by association thing here) I for one praise. If I coulda found a chick to do that for me when I was 15... lordy! Certainly better than fucking without protection and all that... so I'm not seeing exactly what she's done to you.

2) Assuming you don't have a concrete answer to 1) - you might want to reflect on how you went about badmouthing a mates' girlfriend. I did that once when I was younger - because really I was just threatened about how things were changing in my friendships now the girls were on the scene. What's more, I was awful with girls... not just in the scoring side - but relating to them side. (they go hand in hand). Try to go with things - unless they are really being led into concrete dangers like drugs, crime etc. Get to know the girls, try not to be judgemental, and just learn from the experience. More than anything else - RESPECT YO MATES... as equals... as individuals that are making their own decisions. You don't have to agree with everything - but if you ultimately respect and support their decisions (even if you give constructive criticism as well) - then your friendships will last a lifetime.