noun
true friendship; the value of having a friend
Refers to the worth, benefit, or fulfillment gained from a friendship; often used in expressions like 友達甲斐がない (not worth being friends with) or 友達甲斐のある (a friend worth having).
彼は本当に友達甲斐のある人だ。
He is truly a friend worth having.
あんなことをするなんて、友達甲斐がない。
Doing something like that makes him not worth being friends with.
生き甲斐 means 'reason for living' or 'something that makes life worth living,' while 友達甲斐 specifically refers to the value or fulfillment derived from a friendship.
Compound of 友達 (friend) and 甲斐 (worth, effect, value). The exact historical derivation is uncertain, but the word follows the pattern of other 〜甲斐 compounds expressing the value or benefit of something.