Others can speak more on-point from personal experience. These are just a couple of thoughts.
First, your friend absolutely needs a horse-property-experienced realtor who knows the local horse scene, above all else. Whether or not the property sells that way in the end.
Can this property be thought of as a ‘turnkey’ board business? A new owner could just pick up the board business where the previous owner leaves off, without needing to make any changes?
That is, the new owner could make changes, even not continue the board business of course, but if they wanted to keep things as-is, no new investment by them is needed (not for the first year, anyway). No major repairs outstanding, facilities all adequate for current boarder expectations.
The property can be marketed as a rural home that happens to also have a horse board business that the new buyer can keep or terminate. OR as a turnkey board business that also has a residence for the BO. Or both. That’s a choice that your friend’s realtor can help with. It’s likely to depend on the size of the facility and how it is doing financially to earn money in excess of direct costs.
Location will be a key factor.
Good luck to the friend! 