You know I’ve known Lena a long time, I have never seen her like this.
Lena’s probably talked about Kara over a million times with Sam. Seeing that she has no friends as she’s mentioned, and doesn’t know how deal with her feelings about Kara. Sam is trying to help Lena see the difference between, friends and being in love.
There is no other explanation of where Sam got “that’s what friends are for™” reference.