My advice is to keep business relationships based on business.
Its all very well to send a friend a horse but when there is a third person problems may arise. Such as if the horse went lame in their care, or their training deteriorated, which party would bear the cost?
So the best thing to do is try to keep it all as simple and transparent as possible. For example, the new owner may purchase the horse and then send it to the friend, or if the friend is acting as a middle person, they may purchase the horse, train it a little and then send it on. But once a horse starts changing hands prior to purchase things become complicated.
To be honest, I would have had the operation and waited for the horse to recover prior to advertising it, or alternatively not had the operation and offered the horse at a lower price, and then you could just sell it and it could be sent for training without any hassle.
As far as asking for advice online, that's okay, do what you want, but be aware that doing something like that can damage your friendship or business relationship if the other party isn't consulted first.
Keep communicating and be honest and most things work themselves out