There are a couple of ways flowers could influence human respiration at night. One way is if the person is allergic to the flower. This could give them a night of stuffiness and snoring.

If the flowers were relaxing to a person, his or her respiration rate might be slower.

You might be thinking that flowers produce oxygen in photosynthesis, so that would change the person's respiration. However, when there is little light available, flowers won't be doing photosynthesis. They'll be doing "cellular respiration" like us. They'll be taking in oxygen and giving off CO₂ as they burn off the sugar they made and stored when it was light. Either way, there's usually plenty of oxygen around, so that it wouldn't make much difference whether you had a live plant in the room or not.

Do you think that a cut flower still does photosynthesis?