The scene I remember is Mel, Brian and Justin are having lunch (not at the diner) and Mel tells them that if they ever want to have a child that she would be very happy to be the surrogate. I believe that Lindsay has either died or left Mel to be with Sam Auerbach.
I've checked the tags without any luck.
ETA: Found thanks to meta1960. The fic is 'When It's All Said And Done' by Ashleigh Kinnerson.