Two weeks ago I had surgery and gave myself 4 days off dieting. I didnt track, but I know I ate over maint. over those four days and really high carb.
Im currently eating about 600cals below maint. To what extent can I reduce volume and calories, without risking too much muscle mass?
Is this the sort of thing you mean by reduced volume: today was horizontal push/pull, it consisted of
Flat BB bench 4x5 - 225lbs
Incline DB bench 3x6 - 85lbs
Deadlift 3x5 - 375lbs
One arm row 3x6 - 100lbs
For the last three days ive been running calories at around 2400. This is about a 300cals a day reduction from what I was originally eating. My lifts haven't really dropped since the start of the cut, though I started using CM 2 weeks ago and that has really helped.
Does this seem good or do I need to fix anything?
You need food to heal an injury, and surgery is an injury.
Your reduction in volume looks fine.

Getting back to the topic, "I don't want to get too big" came out of my mouth at first, too. I blame the fact that because AAS are illegal, women who use it don't admit it. Hence our collective unfounded fear.
Humour the woman you're training - tell her you'll make sure she doesn't do anything that will make her get too big, and assure her she'll be able to see it coming a long time before it happens.
If she complains about "I don't want my legs to get too big" make DAMNED sure she's dieting, or they WILL get too big for the look she wants. Any woman who says this already has big legs. Cardio won't change this, and neither will lifting. The only thing that will help is a caloric deficit.