Whatever you do or say the goal is not to create a more stressed diver who will just be entering the water more stressed out. Sometimes that is less what you say and more how it is said.
... For the 10 minutes I just spent I can then live with own conscience that I did everything I could to help this person learn something important.
You're on a dive boat ... part of the experience is socializing and enjoying the companionship of like-minded people. Not everyone is built to be nurturing ... but it's in your best interest to do whatever you are willing to help your companions remain safe ... an accident, after all, is going to ruin your vacation as much as it will everyone else's on the boat.
Above all be nice. I don't want to contribute to an already nervous situation. There is always another dive. IMHO. YMMV.
These seem like good ways to respond and mindset when dealing with the scenario. Annoyance, especially conveying annoyance or impatience is probably the least helpful even if it feels warranted.
Some context is always helpful too, was the diver making a little joke about being a out of practice due to the recent types of dive operations they've used? Was this a way for the diver to bashfully seek a little help/reassurance because of maybe some nervousness for whatever reason?
OP didn't mention anything about the level of difficulty of the dives, was this a tech dive trip, AOW dives only or was merely a OW cert sufficient?
Everybody was new at one time, unless this was a boat doing strictly advanced dives, a little patience is probably wise.