If you were married are going with someone for over 10 years, and then found out they were gay, how would you handle it?
You cannot change it, they are what they are. Are there kids involved? Because if there are, he/she will still have parental rights, and you must be Very careful not go make them feel guilty/responsible/bad about themselves!
It would be time to part amicably and both be free to live as they were meant.