Tnx I mean JillKate? She is an airline stewardess, as far as I know you don't find her place of work.
Do you mean Jill? If so, then on a night which Mia picks her up skip the family dinner, and be in the residential district at the time of the meal, once the meal is over, Mia and Jill will arrive and go to their job, and you just follow them.
If you mean Lil, then just follower her after school a few times, you will fail and lose her the first couple, but after that you should find it.