Why not offer $2500 to the grandmother, and then negotiate up to $2600-2750 if necessary? Personally, I wouldn’t expect a horse that was advertised at $3000 to be sold for $2000 unless there was some big issue.
People usually do not negotiate away 33% of their asking price.
Whether or not the horse is worth the asking price is a separate issue. I’d think that a kid-safe horse would be worth the $3000 price-tag. What would worry me more is whether you want to own the horse for the next 10 years. I don’t think that knocking a few extra hundred dollars off the sale price would compensate you if you don’t want to own her for 10 years, and if you do want to own her, then the price seems fair.