Found vomit but unsure which cat

My cats are siblings, borth about 6 months old.

So on Saturday, I brought my male cat to be neutered and on Monday, I brought him home from the vet. In the Tuesday and Wednesday morning, I found vomit.

Male cat wore collar and caged in the night and the female is left to roam freely inside the house.

The femlae one has been looking depressed since I sent the male to the vet, she still play around but now she sleeps further from me and near the door.

I'm unsure which cat vomitted but I highly suspect it is the female. Will separate them tonight, to make sure which one vomitted.

I appreciate any advice and help.