FL I can only comment from what I have seen working as a pilot and also having to deal with this for a group of pilots.
Obviously I will bow to your legal expertise.
But...
There is only a requirement for a basic disclosure for a pilot, no requirement for any enhanced which would disclose any spent convictions.
Taxi drivers I believe require a enhanced because it involves them transporting children and vulnerable adults in an unsupervised environment. Pilots its is very rare they are in that position. It does happen very occasionally with umins but it may never happen to some and those of us in regional ops it may happen but it will be years between it happening.
Also the statement that having a criminal record will immediately mean that you won't get an airside pass is also untrue in the UK at least.
Apparently you can shag a sheep get caught do some prison time and the next day your out you can get a airside pass.
I have worked as a pilot with people that have done time. Varying crimes
Some motoring offenses and getting caught with the wife taking the points right the way through to GBH of a burglar caught in the act. To contempt of court.
The asking questions about a spent criminal record is in the same league as asking a woman about if she is going to get pregnant in the next couple of years. Unless you are in one of the occupations that are listed as being required to have an enhanced disclosure your on dangerous expensive ground.
But to be honest all but one pilot had actually disclosed to the employer anyway that they had been inside. And they have all been good employees and actually I would say above average pilots.
I am not saying that it will be plain sailing for the person but it won't be the huge show stopper that some would like to think will happen.
When working outside the UK in Europe you are required to give a standard 5 year employment history and a basic CRC check as you are in the UK.
Outside that things vary. He more than likely though will never be able to go to the US ever which may give problems training.
Again this all depends what they have put it through as. If the charges don't mention drugs and he has been done on a mil only sounding offense they may ignore it.
I know this won't sit well with some but its the fact of it.
I wouldn't be surprised if in 6 months time if he is training Arabs earning double what he did in the RAF.