My advice would be to determine what is driving you to want to shift from Engineer => PM. If it's purely money, there are alternatives that don't require leaving coding (job hopping wisely, moving into certain types of management tracks, consulting on the side, switching into industries that pay SWEs ludicrous salaries such as quant finance if you can). Similarly if you're looking for more influence/reputation – all of that can be had as an engineer if you're somewhat thoughtful about your path. However – if you're looking to transition out of coding because you're more interested in the business / sales / working with people (which was my story) then it's a great switch.
Good luck!
It's worth analyzing why they plateaued (or why do you think they have).
If they gave up on learning, well that's a sad, but good explanation.
If they found themselves satisfied with what they were doing, that's not necessarily right or wrong.
Do you believe they were in an environment where they are well supported and well managed?
Moving to product is worth thinking about; but it's not a silver bullet either.
While you can pick up experience in tasks not typically done in engineering, the things that can limit a person's career is partially independent of their work experience (in terms of the task they are actually supposed to do).