Did you sign a contract or anything? To be honest, the people you got her from don't really sound as though they have any justification in making you breed from her, yet they would have her put to sleep?

If it is a false pregnancy, there is a strong chance that this will recur again and again. This in itself is a good medical reason to spay that her previous owners surely cannot contest!

I think in this situation anyone would have 'weakness' so I wouldn't worry.