He sort of, fairly timidly, questions it, with the flimsiest of legal analogies: the legislature has to agree to military missions, e.g. participation in UN missions, and thus (?!) something something Merkel. It's a complete non-sequitur.
Now if you look into this Kopp Verlag, you will find that the Verfassungsschutz is keeping an eye on them because they might be spreading right-wing ideology which seeks to overthrow the constitutional structure of Germany.
Interesting source for claims of unconstitutional behavior by Merkel ^^.
Now I'm not saying that any of this necessarily makes what they say untrue, but I guess your choice of sources makes one thing clear: Nobody here will be able to convince you.
Here's the paper: http://bayern.de/Gutachten_Prof_Di_Fabio
Here's a rebuttal: http://verfassungsblog.de/dem-freistaat-zum-gefallen-ueber-u...