Frozen embryo's have a shelf life (and I detest talking about life in this way, but there is little else way to say it). Eventually, they become unvioble.
There is no easy answer to this as it is difficult to see how the end justifies the means in either case.
The problem is that IVF is very immoral because it removes the unitive from the procreative in making new life just as contraception removes the procreative from the unitive.
In the case at bar, the new life is already here. The gnat straining of Humanae Vitae doesn't apply, and isn't going to take care of that camel, the viability of the embryos at present.
Nor is there an ends justifying the means dilemma, as the lives are already at steak. Too late to debate whether IVF is immoral (though not too late to debate stopping the future production of embryos, or subject them to restrictions).
I am aware that many who follow HV see a woman carrying one of the frozen embryos akin to murder. It is, however, more akin (at least in this circumstance) to donating a kidney.