Curious one------to answer your question as succincly as possible.
Mil. bases in Germany used to employ considerable numbers of the local population so no problem there. Also, quite possible the German was so attired although wearing such was not commonplace I agree.
Now to the rest of the claim. IF your friend was doing a ground run ( a ) the aircraft would have been a located well away from any potential hazards in the form of passing personnel---or should have been. (b) he would not have been alone and would have had a starter crew to supervise the run and the safety aspect----that would be two, maybe three others at least. The chances of anybody getting that close to a re-heat run would, therefore, be zero---other that is,than those who may have been working on the engine of course.
The alternative is that your friend carried out a ground run and the jet efflux, did blow a passing civilian over----that said, he should never have been allowed that close on the first place.
All told, your friend has a emanates a distinct odour of class A B / S--and if he did knowlingly and deliberately carry out a ground run and was aware of a passing non relevant personnel and continued with the run with the intention of blowing this person over----he has A£SE%%£E stamped all the way through him.
I would change your social circle pdq if I were you.