How do you unlock Lin's footjob?
What do you mean by unlock? Once you have gotten that once you know how it can be had, there is no unlocking.
Spoiler
Meet Lin in the kitchen when she is working there. Ask her to wear the sexy apron. I guess there is a minimum relationship level requirement, or maybe some attribute needed, but I do not know more.Eventually that fetish is available, just continue playing the game.