If as you say you put up the entire down payment and you are both on the mortgage, then you may have what is called an "oral general partnership" with regard to the house and the mortgage obligations. That means that unless she's contending the downstroke was a gift of some kind she would theoretically owe you 1/2 the "equity" back and you'd owe her 1/2 of the mortgage payments to date, in which case you'd also split whatever the paydown of principal was over that time period, which is probably negligible. At the same time in part depending on how you were named on the deed you are theoretically entitled to occupy the house as an owner too, so that should also work in your favor, because if she is occupying the whole house then theoretically she owes rent (or use and occupancy) on the ownership interest that is yours, and I would argue that her making mortgage payments in part represents such compensation and not entirely hers as equity "appreciation". However, since she has now gotten an attorney involved and while you can still try to work it out with her directly my suggestion is to also get counsel.
Assuming you were both on the title, what you would probably be facing is a partition action. The goal of the partition action is the divide the property between two people accounting for what they've put in to it and what it's worth now. It often involves the sale of the property, or one party buying the other out.
Without knowing more facts about the numbers involved, it's difficult to say where things would play out between the down-payment, the mortgage payments, and the current equity/value of the house. I'd suggest you sit down with an attorney if you think she's going to seek payment for you or if you think you're entitled to some money and want to get it from her.