Thoughts...
Did the flight your SO was rebooked on lack Economy Plus availability? Wondering if that played a factor.
That having been said, I've not experienced being split from my travel partner if we're on the same PNR. However, what may have happened is that when you checked in, your PNR's may have been split for upgrade list purposes, which effectively would have made you both solo travelers. See:
http://www.flyertalk.com/forum/unite...eeds-stop.html
As for rebooking on other airlines, UA has done this for me many times. I've been rebooked onto DL, AA, and US (pre-merger) in the past.
Re: the boarding pass issue, you'll want to make sure that the AS flight was properly ticketed. I've been in a situation before where the UA CS thought she put me on US (before the AA merger), but there was some snafu/miscommunication/mistake which resulted in US never actually ticketing the flight. I can't recall if it was b/c UA didn't properly release or endorse the ticket over, or if US never approved (or whatever the proper terminology is). In the end, it took a call from the UA ticket counter to US to get it sorted, and I think it was US's fault.
If the flight was paid for and not an award booking, also be sure to ask for original routing credit so you get your miles, segments, and dollars.