Are restrictors in shower heads bad for instant hws

My Neighbour fitted a new shower head when the old one died, it even came with a small spool of thread tape. Guy at the plumbing store said it might need to have the restrictor taken out if it didn't work properly with his 7-8 yr old instant hws. It was a bit of a pia to get used to but after 3 or 4 days he and wife had it down pat. A few months later the shower taps start dripping so wife calls a plumber, "while you are at it take out the restrictor" she asks. "I would like more flow".
Job done and the hot water is not so hot any more, plumber blames husband says he wrecked the hws by fitting a shower head with a restrictor, "too much back pressure wrecked the diaphram" says plumber. Wife is angry says husband is an idiot.
Husband argues how can it be, isn't a tap a restrictor, you turn it, it restricts the water to the point of stopping flow altogether, how can a restricted shower head do this but a partially turned on tap not do this, plumber explains you turn on the tap, get full flow and bang water hits the restrictor and wrecks the diaphragm/regulator, simple.
Question is the plumber right ? logic tells me to agree with the husband. If the plumber is right then every person in OZ who turns on the hot tap and then uses it to slow down the flow a bit (me included) is wrecking the hws. My thoughts are that maybe the neighbour's hws needs servicing and with the restrictor taken out the increased flow through the hws has given the water less time to heat up and so the hot aint so hot any more. Before he left the plumber handed over a quote for new hws because the old one was wrecked and not worth fixing. Is the plumber right ? or is he just looking for that extra job this month.


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