Preserve the integrity of the game and play to win.
This wasn't a cash game.
Soft play between 2 opponents when you're the 3rd wheel 3 handed is completely inappropriate and in violation of the spirit of the competition. Poker is difficult enough without 2 players essentially colluding against you.
I know it isn't a fair comparison, but I run a league where there are multiple married couples who regularly play with us. If they ever soft played against one another, they'd be banned.
The bottom line there are no friends when you're in a hand against one another. Granted, it's easy for me to say that and I'm sure it's not easy being in the final 3 of a major event with your fiancé. But I hold the position that you must set aside all personal feelings and relationships while you're in the game, and there's no hard feelings after the fact.
PS I also never let my kids win at anything. It means so much more to them when they beat me knowing I was trying my hardest.