You'll probably have to ask her about that.
In all seriousness, I'm having serious trouble imagining a situation where a casual acquaintance or stranger would, upon reading your book, A) recognize that random statement they presumably made years before the book was eventually published, B) be angered enough to pursue legal action over it, and C) be capable of proving your wrongdoing in a court of law without it being thrown out as the mother of all frivolous lawsuits.
IANAL, so take this with the appropriate grain of salt, but I don't think random utterances are protected by copyright. I certainly hope not, otherwise there are a whole lot of people in my life who have grounds to sue me.
I mean, if its someone you know well and want to maintain a good relationship with, its probably polite to just ask permission, at least if its anything particularly memorable or creative. But if your poaching lines from, say, a conversation you overheard random strangers having on the bus, I can't imagine any consequences, legal or otherwise, arising from that, unless they happened to be reciting their doctoral thesis or social security number at the time.