The way I see this, you would be paying 6 months of boarding for a horse you cannot do anything with.
I fully understand that this trainer doesn't want to risk her pregnancy training an unbroke warmblood. Who could blame her? There can come times when the horse needs some "understanding" moment, and those can be pretty intense. A good trainer should be able to get on the horse herself and fix the issue. She cannot obviously do it if she's heavy with child.
That said, it is not really your business if she's pregnant. You don't have to pay for her horses, or for the baby's diapers.
If your daughter wants an unbroke horse to train with this trainer, look for one in october or even november, when the trainer has fully recovered.
If your daughter can settle with an already trained horse, look for one.
If this 6yo you found is the love of your life and the sun in the sky and the moon at night, look for another trainer.
I really don't see why you should pay for 6 months worth of boarding for no reason other the personal problems of your trainer.