Whenever she starts up, bring up her fiancee. Ask about him and what he's like. Ask about her upcoming wedding and things like that. Keep reminding her she is not available and neither you or Bill are interested in anything beyond office friendship. Be polite but don't encourage her by asking her to lunch with you or go any where else with you.
There was lots of trouble with the office I used to work at when people wanted to be friends outside of work. People told things about others that was no one's business. (and even made things up) I since made it a policy to be nice and friendly to people I work with but after hours are off limits. I was there to work not socialize. It kept my life a lot simpler.
Oh, and if Bill is married or has someone special in his life, he should talk about them ad naseum when she is around.
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