I think it's a little weird that she asked you to write a letter "from her." She should have access to a computer, at the library at the very least. But, if you are OK with writing the letter, I'd write the letter along the lines of 'I noticed your horses are in poor condition' with a few details as to why you are drawing that conclusion, and offer to help work with the owner to determine a proper feed and care regimen. Take it to the lady from the rescue and HAVE HER SIGN IT before you put it on the stall door.
An even better approach would be to mail it via certified mail (the BO should have the address) so that there's a record of the correspondence if it ends up going to court.