Father in law person:
I'm retired, and don't look in often. Good for you for helping the guy out, it's hard to know where to look.
Everyone in the business has their own perspective, as I do.
In some cases, opinions tell you more about how those with vested emotional interests think things should be, rather than how they are.
Many many many people will tell you a load of nonsense about needing a lifetime of experience. It may help, or not, but isn't necessary and doesn't make you the best person or engineer or businessman. You only have to be good enough.
My own route/experience is way off typical, so a different angle of view. Routes have changed over the years so I may be out of date.
Plumbing and gas are separate.
You don't need any plumbing qualifications to become gas registered.
Some plumbing knowledge may be useful, some not. If all you ever do is service boilers, you don't need much.
You used to need to do a gas qualification, which lead to a teacher at NESCOT in Surrey inventing the "Guild of gas fitters" qualification, which did the job. That avoided slogging through pedestrian NVQ plumbing, which had gas bolted on.
I did full C&G though as an underling, sheet Lead roofing and all.
I was an ACS gas assessor/examiner for a few years while I was still installing, so I saw a few hundred blokes come through. (And 4 women).
As a beginner, you also require experience, to satisfy the ACS gas criteria. Nearly everybody lies about the experience.
You used to practically have to, so accepting lies became the norm. Maybe that changed. (At one time you had to have experience of fitting some ridiculously large number of gas hobs under supervision, for example.)
There are lots of short cuts, so look for them.
The worst individuals I encountered while assessing, were "limited" old soaks with a bloated sense of of their own value, insecure from only ever having learned about one thing in a lifetime. Beware of advice from those.
It's not hard, but it all depends what mental abilities/aptitude you have. Anyone with a science-ish A level could easily learn all you need to know to pass the "gas exams" in a weekend. As they could for NVQ2, with an extra day for NVQ3. There's less in "the gas", than a modern GCSE, say. (As hobby I now teach GCSE and A level, (electronics and physics) so I'm up to date for those.) If a science GCSE would, or would have been, too much of a stretch, then it would be a struggle to get through the gas training & qualifications. Most of it's applied common sense though, so it's understandable.
ACS centres have a vested interest in passing people, and they know that they're only checking a sample of the material. They never have enough time to do all that they're expected to either. We tried with every new set of "requirements", and found three different Boards', sets, utterly impractical to test entirely.
If someone knows and understands the basics and can look stuff up, and they aren't too arrogant or thick, they pass. They aren't going to be dangerous unless they're careless.
A newbie would need to know a list of things, but it wouldn't cover more than a couple of sides of tight 10pt A4. Learn it by rote. Let's not call that hard?
If you fail, - no big deal. You can make the cost up on the first boiler or two you fit.
If you want, tell him to fit bathrooms in the mean time. Easy money in Surrey if you're reasonable at it.
Hope that helps, just a bit.