I would do it if they can get by with just carry on bags. Talking to anyone at a check in desk would be the biggest risk.

Also, have a 'story' already made up, like "he is flying in on another flight and we will meet him airside", and/ or "he was flying in on another airline and his flight was cancelled".

And, indeed, maybe two one-way's are better. I would worry that if you don't show up for the outbound, that they might cancel part or all of the return.