Here is the law that pertains to boarding horses and not getting paid: Massachusetts General Law Part III, Title IV Chapter 255 Sections 24,26,29
Chapter 255: Section 24. Domestic animals; care and custody
Section 24. Persons having proper charges due them for pasturing, boarding or keeping horses or other domestic animals which are brought to their premises or placed in their care by or with the consent of the owners thereof shall have a lien on such animals for such charges.
Chapter 255: Section 26. Enforcement
Section 26. A person who has a lien, which is not described in sections fourteen to twenty-two, inclusive, or in chapter two hundred and fifty-four, for money due to him on account of work and labor, storage, care and diligence, or money expended on or about personal property under a contract express or implied, if such money is not paid, in the case of a lien described in section twenty-four, twenty-five or twenty-five A within ten days, or in other cases within sixty days, after a demand in writing delivered to the debtor or left at his usual place of abode, if within the commonwealth, or mailed postpaid to him at his usual place of abode without the commonwealth, may bring a civil action in the superior court or in a district court within the jurisdiction of which the plaintiff resides or has his usual place of business to have the property sold to satisfy the debt.
Chapter 255: Section 29. Order for sale
Section 29. If, upon a default or a trial it is found that a lien exists upon the property and that the property ought to be sold for the satisfaction of the debt, the court may make an order for such sale, determine and record the amount then due and award costs to the prevailing party. Any proceeds of the sale remaining after satisfying the debt, costs and charges, shall be paid to the owner upon demand.
The only trouble with this is that you may very well get stuck with the horses, and you will definitely get stuck with feeding them and caring for them until the owner comes up with the money. If you have to sue her in court it would be expensive unless you felt that you were capable of representing yourself. If she wants her horses, though, it will not come to that and she will pay rather than take a chance on letting the horses go.