Word is from a couple UAL f/a's i asked when I tried for this back in May, the junior base is BOS, but when I asked someone at the prescreen interview they were hiring for BOS, NYC, IAD, ORD, DEN, and LAX. If she really wants SEA don't count on it from what I heard its super senior all the ladies that were around when UAL created F/A's are based there. Like most airlines however, you don't get to pick your base, and you probably can transfer after a couple of months or so. The interview process for them is rather different compared to CALs, she'll get brought to CHI for a prescreen interview done with a hiring agency in downtown (real nice people, the building is a bitch to find however), if they have an interest, the deal is they write you back in 2-3 wks (never got this far) then you're interviewed by United personnel at their inflight training center in Elk Grove,
and if they think she's the woman for the job, they'll notify her when that 7 wk F/A training class starts.
Wish your friend the best of luck
-Paul