An elderly friend has moved into a new retirement home. she said that the
shower keeps going cold soon after she gets in. I don't think it will be a
shortage of hot water. She describes the shower valve as chrome with two
knobs - one for temp and one to turn it off.

I don't have any first hand experience of thermostatic valves (I have a
pressure balancing type)but I suspect she may be doing the typical "old
person" technique of turning it up full to bring the heat through - and
this might be activating some sort of anti-scald device.

I imagine the builders would have fitted a safe unit.

As she is 20 miles away, and it blaming the builders and the modern world,
I would like to try and make an appropriate suggestion before she gets too
wound up.

Do such showers have a feature that is likely to respond to a high temp by
shutting down the hot flow?

I feel inclined to tactfully suggest she sets the temp lower before turning
it on.