First of all, I don’t think that LD’ing is addictive, because you generally have to make efforts in order to obtain lucid dreams, and when you relax these efforts, LD’s disappear.
About Paul Tholey article, he said : "We also believe that these experiments might lead to psychosomatic disorders. But he has never published afterwards any article concluding that his suppositions were justified. Moreover, Flo (a great LD’er) has made a lot of experiments with agressions against DC’s, and she has never experienced any side effect.
Feeling you’re unable to wake up is sometimes experienced by beginners. It can be frightening, but it doesn’t correspond to any real danger. If your friend told you this experience, it’s because he has woken up, hasn’t he ? 
When you start a new activity, you’re frequently stressed or frightened. It doesn’t mean that this activity is psychologically unhealthy.
From my experience, it never happens. This method has been created by Paul Tholey, a German psychologist, and he has never related such problems with it.
These questions have been developped in the Lucid dream dangerous? topic, and I never found a conclusive issue against LD’ing. IMO, it’s normal, because lucidity (or consciousness) is the natural state of dream.
Paying to much attention to dreams can be dangerous for psychotic people, but that’s the case for ND’s too, not only for LD’s.