I'm so thankful I haven't gone through this one yet... I hope I never do.
Step 1 would be to contact the executor with copies of the contract in tow. (Sounds like you've done this). With these documents I would also write a letter with condolences and with out mentioning the possibility of repo explain that the horse has farrier, shots, worming or what have you coming up and your concerned for the horse's care.. offer to over-see these things and they can just include those fees with the board. After-all I'm sure the deceased would want her horse to be care for.
Also ask what their intentions are with the horse. Do they plan to sell him? Continue boarding him? If they want to sell him offer your services so they don't have to deal with it. Sell the horse take your board and commission off the top. You should know what their intentions are so you can prepare NOW rather than later. If they intend to keep the horse, suggest a lease option to cover the horses expenses for now.
If the funeral is past, I think its safe to persue your business with them. After-all it's a loss for you too... I would be immensely sad if one of my boarders passed away even some of the crazy ones. Not to mention, you have bills too.. Business doesn't have to be cold and in-personal.
~ Starline Stables ~