stretched timing belt?

A friend of mine has an Astra. It came up with error code P14 (exhaust cam
advanced). I replaced the camshaft control solenoid, and it was okay for
a short while.
But then started losing power, stalling and throwing more error codes.
So she took it to a dealer workshop. Their diagnosis was "stretched timing
belt". I haven't heard of this before. Usually just hear about them
snapping and wrecking the engine, or some teeth breaking off, and it
slipping out of phase.
I guess if the belt became longer, it would be benign at constant speed,
but then go out of phase when accelerating?