My horse is kept alone, and he does show signs of missing the company of other horses when he sees them whilst we are out riding. However, he does have almost constant view of a pony that lives next door, and is often put into the paddock next to the pony for a few hours a day - yet he and the pony never have anything to do with each other! I would prefer him to have a friend, but as I agist him it isn't really plausible. He does have the company of a mob of cattle in the next paddock who often hang at the fence with him, though it isn't the same.
Keeping horses alone isn't optimal, but if you need to do it, don't feel too bad. Just keep an eye on how your horse reacts though, as some fret terribly.