Sorry, but from what I've read (in massive amounts of time on IF boards), it's pretty unlikely.
The first time I got pg (it ended in m/c at 8 weeks) I did have a negative HPT the day my period was due. But it was a Clomid cycle (not IUI/IVF), so my doctor didn't do a beta right away. I didn't even realize I was pregnant until a week later. As far as I know, everything was normal until 6-8 weeks or so (i.e., it wasn't a chemical pregnancy). So the fact that it ended in m/c doesn't necessarily have anything to do with the negative HPT.
I also had two positive HPTs the night before a beta. One was an IUI cycle and turned out to be a chemical pg -- the other was the twins (which was a much darker positive).
In general, I'd think if there's enough hcg in her system to be a viable pregnancy, it's probably enough to register on an HPT.
As far as what to say -- I don't know. It's hard for me to remember what it felt like to think we might never be able to have kids (though I do remember how infuriating it was that my sister kept getting pg without even trying
). I think you might just need to give her a lot of space.