It's hard to say without knowing the possible ramifications of this person being there. Sounds like she once worked there and now doesen't, and there was an issue in her leaving, or something of the sort, prompting her to phone you with a heads up she was coming back? Of course I understand if you dont want to air all the details of someones situation online.
I think I would just go with my general rule, for when those not family or not close friends lie or otherwise betray. I keep it polite, but just don't trust them as much or seek closer friendship with them.
Your story reminds me of when I was working at a particular place... Everyone in the store knew the manager had parties and this co-worker and that co-worker over in the evenings to down as much brew as they could manage. I was invited and went a time or two, stupid as it was.
All anyone could ever talk about the next day in the store was how funny they got the prevous night.
So this one day, I started work early in the morning.. and late in the afternoon, the other shift leader was supposed to come in. He didn't come in, and I called and called, as I was stuck working in his stead. He finally picked up to tell me, in a laughing tone.. that he was really sick and couldn't leave the toilet. I called the manager as well, left messages... and he called me back an hour later to tell me the guy was having family issues... while I heard music in the background and the co-worker yelling "Tell her I was in a motorcycle accident" ... laughter.
So I called the owner of the store.. and who had to come in to take over for me, but the lying manager... swearing up and down that he was just telling me what the co-worker dude had called to tell him. Yeah.
So the next day at work.. everyone is in an uproar over all the hilarious things the co-worker and manager had said and done
I still worked there.. and was polite every day after... but I made it clear to the manager that next day back, if that ever happened again, I was walking out the door when my shift ended and locking up behind me.
Sorry to hear you're upset Rammy. All I can really think to say is that I've had bosses in the past who are for whatever strange reason, very secretive about things. Maybe it makes them feel important or official, maybe they think as bosses, they shouldn't be discussing these things with employees, I can't say for sure. I had one boss years ago, well two actually, they were partners, who had plans to go out of business and didn't tell any of us for months. I ended up finding out early, because I was kind of close with my boss and I also did the ordering for our stock and noticed he wasn't refreshing the stock as usual. So I confronted him privately and he admitted that they were closing the New York store (where I lived at the time) and moving to New Jersey. It was a good distance away, so it wasn't like we could all just transfer there. He asked me to keep it quiet and I did, but it turned out that he waited to inform everyone until the week they closed, which I always thought was horrible. But I got sidetracked with my own story, sorry. My point is bosses can be strangely secretive, and my guess is that my boss probably would not have even told me if I hadn't asked, and I was close with them. All I can suggest is to try and not take it personally. Or, if you are taking it personally, and this is very serious to you, then find a new job as you mentioned. I'm sorry you feel betrayed and I can only assume that life has better things in store for you, because you deserve it.
That's one of the things I like about my job. I don't work in the office and don't get embroiled in the drama. Finding another job is a good idea. When it comes to family people do stupid things. Like replace hard workers with lazy family members because the grief factor at home is too high if they don't. That's why most places have nepotism policies. Of course in a small self owned business you won't see that.
Good luck. Try not to let it get under your skin too much.