Your threat title: I should have let him fail... ?!
The way thats written sounds like you have the power to do that, and that he wasnt capable. You've assumed so much. As an interviewer I see how often candidates misinterpret the intentions of any given part of interviews.
To work in a secure unit, trust and honesty would need to play an essential part in the selection process, and you cheated.
People get deliberately pitted against each other in vivas to see what happens, to observe the interplay and who's prepared to do what.
You've essentially cheated. You've been to uni, which means you have done more than enough exams, and are intelligent enough, to know how they work, and what the rules are when sitting tests. This is not a matter of you helping your children to do homework, this is a grown man in competition with you for a lucrative post which could lift you and your family out of debt! Fight for this!
Test situations are observed. Especially where more than one candidate at a time is involved.
This man may be far more capable than you at this job, this man may also have been a stooge, because of the environment you were going into, and I would say based on that alone, the role wouldn't be suitable for you without some significant awareness training.
You have made some very discriminatory comments about the under 40's! And sweeping generalisations about the over 40's! Neither of which are true.
All in the name of kindness and guilt to control this mans chances, which you can't have control over. You were not interviewing him, so you can have no idea.
Inside a confidential space with an interviewer people present very differently than when amongst their fellow interviewees.
I really hope you hear in the positive, as you need this job so badly, but protect yourself in future and be aware of the prejudices you have shown through this interview process. It will help you to focus on what you need to do, and not others.
I think most of us would have given to odd spelling or something, but you essentially worked together to complete his test. Didn't you think you were cheating?