Thanks for the replies. I guess that means you can't have your cake, and eat it too. So, qualified friend cannot use pre-check.
Concerns a knee transplant. Metal detector leads to the patdown, shoes off, re-check; only benefit may, in some cases, be the shorter line at pre-check.
I honestly don't think this part of the policy has been thought out.
I would suggest that someone in this situation explain to the first available TSO past the TDC ('moat dragon', 'traffic director'). It does appear that they are making efforts to ensure that there's no 'crossover' between Pre-cleared pax and regular pax (makes sense), but it is also possible that a Pre- WTMD alarm will receive a less-invasive grope or an escort through the nearest NoS (hopefully while allowing the pax to keep an eye on his bags).
Perhaps someone who has Pre- and has alarmed the WTMD will weigh in here and describe his/her experiences - ie, what does TSA do to resolve the alarm?