its unfortunate but it happens...a girl comes between two good buddies...but you have to move on. as has been said if you have no written agreement you'll just both have to sit down and work it out. one buys out the other ( and hopefully won't wind up in litigation which would be sad.)
if you can't agree, it has to be sold.
sad but any partnership is like a marriage. if unhappy differences occur they have to be worked out or its at an end.
sorry but my guess is you already know the reality.