Woah - slow down! Sounds like you've got yourself into a bit of a tangle.
First off - don't do anything that you don't want to.
And second - is revenge really the right way to go? Dealing with ex's is often complicated, but there must be a better way of keeping in touch with him than getting with his best friend. Obviously, no-one here knows the in's and out's of either relationship, and can't tell you exactly what to do, but it seems a bit like you've got really mixed up and have made a few rash decisions. Maybe it's worth sitting down and having a good sort out of your own head, decide what it is that you really want. Make your current boyfriend aware that you're uncomfortable with what he's asking you to do. But please don't string him along, that's a bit unfair. If you're only with him for the sake of revenge, then it's not fair. It'll only nd in tears if you carry on like this.
Maybe this is the perfect time for you to decide exactly what it is that you want.