As my Grandfather has gotten to be... Well 91 this year... GO Gramps!!!
Anyways, My Grandfather had horses... Slowly they have died off on him. Cocoa has been alone for the last 15-18 years...He was the last baby of the horses that he once had. Seems to me Cocoa was about 4 when his momma died and has been alone ever since...with the exception of a few head of cattle in the first 5 years but definitely alone for the last 10 years. He's been fine. If it's only for a little while I'd say it would be OK.
Of course horses like to have a buddy...but they can do without.