"and wants to deduct the cost of the trailer as advertising"

I love this and ran into it all the time in the classroom. My answer was, "Yes, the SIGN is deductible as business expense."

The function of the trailer has nothing to do with insurance. It's not business related unless the horse activities are treated as business, and then it's an Asset.

"and she would like to write off "

Yes, of course she would like to. The attorney might like to, as well. Why not both? Ha ha ha.