I am not a plumber but I have a decent knowledge of the BASICS.
As far as I am concerned, plumbing is still a trade and should ever be regarded as such.
Being able to put a few push-fit joints and plastic pipe in a domestic property doesn't make you a plumber and there is a hell of a lot more to plumbing than doing a few pipe runs.
Professional Plumbers have a sound knowledge of a very wide variety of related matters. To name but a few, does your friend have an intimate knowledge of building regs, water regs, pipe sizings, flow rates, etc etc?
Has he got central heating? If so, is it all electric? If not, does he have an intimate knowledge of boilers//cylinders, pumps, tanks and related pipework? Whilst it may be a Corgi//Capita [!!!] who does the gas work, plumbers do the majority of the pipework and fittings.
No disrespect to you nor your friend. I've done a bit of pipework myself. However, IMHO, there is NO substitute for an experienced professional who can truly be termed a Plumber with all the knowledge and expertise that label carries.
When [if] something goes t*ts up, I presume that the professional will be the one who attends to rectify the errors//problems.
In the words of Alexander Pope,
"A little learning is a dangerous thing" [oft misquoted as 'knowledge']
Of course, that's only MY opinion. I take my hat off to all true professional tradespersons - many of whom give their time to also post on here and impart their knowledge to those of us who lack the experience and knowledge they hold.
As for money, I refer you back to one of my earlier posts about charges for professional services and the tale of the bank vault.