Fiction takes inspiration from real people and events all the time. Think of the book/movie To Die For, or the many tell-all romans a clef, or the "ripped from the headlines" episodes of Law & Order. Remember when Nicole Richie wrote a novel that was supposedly ripping on Paris Hilton? (Or so the tabloids claimed, anyway.) Etc., etc.
Common as it is, authors do occasionally get in trouble for this, especially when the inspiration is not a public figure, is easy to identify, and might have a damaged reputation as a result of the book. I read about one case where the author fictionalized her experience with an unconventional therapist. Before undergoing therapy, she had made some sort of non-disclosure agreement. Even though she changed the practitioner's name, this resulted in a successful (I think) lawsuit. (I found this account on a publishing lawyer's blog, which might be a source to start with, and then consider consulting a lawyer.)
The best defense is fictionalizing as many incidental details as possible, so that the story is distinctly yours and does not come across as an invasion of the person's privacy. Writers turn their real friends, family members, and acquaintances into fiction all the time. Hard feelings result, but actual lawsuits seem much rarer.
As for agents, I wouldn't volunteer the info unless they ask what inspired the story, and even then, be vague and avoid naming names, unless the person is famous enough to be considered a public figure.
It's always weird seeing the standard disclaimer ("any resemblance to persons living or dead is coincidental") on works of fiction that were obviously inspired by real events. But I think the key is plausible deniability and a strong creative component. I'm far from an expert on this, though.