Girlfriend and I played a game where she tried to trash 2 estates and a squire with forge to get a Gold (which should be possible right?), but it would only allow her to get a card costing 3.
#107931144 on Tokyo
Squire reads, under the line:
When you trash this, gain an Attack card.
In this game, the attack cards were Enchantress and Swindler, both costing $3.
So what your girfriend saw was not the prompt for Forge, it was the prompt for Squire's trashing. After choosing to gain either one, the next prompt is about choosing for Forge (between Gold and Captain).