I have little NO experience with GM'ing DFRPG, but I with what I know, I think you made a fair deal to them. Unexpected things always happen, this is one of them, you deal with it as best you think you can. Consider this, though: they sent her a note/message after finding out where she could be contacted, but how reliable was the messenger? Or the message itself? Or perhaps Estelle has an aspect STUBBORN STREAK, or DON'T TELL ME WHAT TO DO! (or something like that)
From your post, I couldn't really make out if they had been involved for a longer period of time, but as they didn't know where to find her, I'm assuming they aren't really an item yet. Imagine if someone you met only a little while ago sends you a message, telling you not to go some place at a specific time. Imagine your surprise at being given that message, while you hadn't shared your address. Imagine being suspicious or curious. You could come up with lots of reasons for Estelle to be there, whether or not the message arrived.
If you're afraid the PCs might feel their efforts are wasted, you can handle it delicately, I think, by having her come prepared. So, while she still is a liability, instead of hiding behind the PCs' backs, she brought a weapon, or a bunch of friends and they help to scare of whatever is causing trouble that time.
I don't know how exactly the players handled finding out where they could contact her and what the message entailed, but if they weren't specific enough with either, I feel it would be somewhat arrogant to assume that 'we sent the message, so she won't come'. Free will and all that.
To recap: I think you gave them a fair deal, but I would have twisted it a bit. If they pay off Estelle's compel, an entirely different innocent person could enter the fray, for example.