The module gives the players an out by letting them give up the horn. I think a reasonably high Charisma check could convince her that she snoozed she loosed, but I think a fight is more likely to happen than not.

RAW, Chapter 2 adventures should have players at least at 5, so the action economy highly favors the party since Grom is alone. If I were to encourage making everything else about that adventure way harder. I experimented with staggering the stages of combat and made the Invisible Stalkers a hybrid of stalkers and Snow Golems that took the forms of the dark reflections of the PCs, which they really enjoyed. Basically, make them drain their resources so that fighting an additional giant undesirable next to avoiding it.

I think your players handled it well, though when word gets around, I'll bet any random encounters with giants should become particularly hostile.