eleventh hour friendship 

n., a friendship formed at the last minute, usually because a common means of communication is going to expire. Eleventh hour friendships are commonly formed at the end of grade school, high school, college, and periods of employment with a specific employer. The term comes from the "eleventh hour", i.e. the last possible time.

Joe: "I didn't know Bob was friends with Tim. They never hung out in high school."
Jack: "It was an eleventh hour friendship - they literally only got to know each other after graduation."

eleventh hour friendship by Col. Hans Landa December 28, 2011