Does she have the claim check that the bag was checked under, showing the other name and destination?

This happened to me on UA 10 years ago, and it doesn't sound like they're handling it any better now. In my case I was checking at BDL for SMF, while another woman was going to SFO and my agent grabbed the wrong tag. They called down and claimed that the baggage handlers would fix it; 3 guesses whether that happened or not. I should have insisted on being given the woman's claim check, or at least her name. Fortunately I'd seen that her destination was SFO.

I dutifully filled out the lost bag form at SMF, and explained exactly what had happened. For a week several agents claimed they had sent a "message" to SFO to look for my bag, with no response, and since I didn't have the name on the actual tag they couldn't find it in the scan database of course.

Fortunately my then-GF lived in SF, so the next weekend we took a trip down to SFO, and found my bag locked in a cage. I suspect I never would have seen it otherwise, as the baggage agents there clearly had no interest in matching up orphan bags with the info that an extra bag had been sent to them.

If you have reason to believe the bag is actually in SEA now, I'd suggest going down there in person. That may be the only way you're going to get it resolved.