Sounds almost identical to a situation I came across on LHR-JFK earlier in the year.

Couple in their late 50s/60s I'd guess, husband gets pulled over for secondary at the gate and I board right behind the wife. I'm in 64K and she says to the welcoming FA upstairs, "My husband is Gold or whatever, but we're not sitting together" and demanded people be moved about. I have my large rollerboard on the storage bin, already open as I'm about to change in the toilet and I hear the FA say, "He looks like he's already rather settled". The guy in either 64A/B shows up and the couple plus FA manage to sweet talk him into taking 64J. He didn't seem too bothered by it but given this woman's attitude I would have expected an almost identical response to yours had he not helped out.

My only guess in your case is that they couldn't pre-assign upper deck seats together so found it better to pick one up there and then hope someone who doesn't care about their seats was willing to swap. Similarly in my case I was pretty certain they took any two upper deck seats as a pair wasn't available and banked on an easy switch.