I would bet anything that she succeeded not just because of her people skills but because she was a quick study and learned what it took to be a good engineer.
More learnt to recognise what made a good engineer, than learn to be a good engineer. Her standard intro to new staff who asked what her tech experience was went something like this: "I was a trainee programmer once....I totally sucked at it."
The only guy who tried the "If you've never been a programmer how will you understand the skill and quality of our work?" BS got the following answer: "I press the buttons and it works, you're a genius, I press the buttons and it doesn't work you're an arsehole - there's nothing in between." He lasted 3 months.
That said, it seems like the easy solution would be to down-promote this person one final time after reviewing their performance...
Now let's talk about this fabled review of performance: You're the newly promoted manager, and unfortunately you're crap at your new job. I'm the manager that promoted you into that new job - so by definition I'm crap at my job - and I'm doing the performance review.
The great conspiracy of mediocrity means that the unspoken sub-text of the performance review is: "We both know in our heart of hearts that we're crap at our jobs and if we could have our druthers, we would both like to go back to what we were really good at. But we're both trapped, we can't publicly admit we're crap, so we'll just continue to mosey along being mediocre at our respective jobs. Be a good chap and don't rock the boat, and who knows, hopefully there's another mediocre uber-manager who will promote both of us one last time"