Funnily enough, Geedouble, I did EXACTLY what your fiance is doing. You can do it the honest way, like Lola suggests, and that's totally fine.
However, I did it the easy way. In May 2004 I purchased a return ticket to Manchester to stay until August 2004. Upon arrival I merely said that I was visiting friends for the entire summer and traveling around the country. No questions asked, no problems.
I then lived with my partner (he supported me as you will be doing - that and we lived off some of my savings), I didn't work, and I returned back to the US in August 2004. He then flew over with his family in September 2004 and we got married (he didn't tell the IO that he was getting married - there wasn't a need as we had decided to settle in the UK so he didn't need a fiance visa).
If you feel more comfortable with Lola's suggestion, then go for it. But as long as your fiance isn't going to be working then she isn't breaking any rules in visiting you for 3 months. I mean, you are 'friends'.... right?!? Haha.
Goodluck regardless!