Matthew is extremely naive and may find that his skills are neither wanted or needed by the YPG at the moment. His motives are however very noble. My wild guess is that PYD/YPG may take Matthew on for propaganda purposes only and that they won't have him anywhere near their front lines (or at least not for very long) . Indeed they have to do this to save face with the Kurdish press.
Matthew is 43 years old for starters. His language skills in Kurdish, Arabic and Turkish may also be lacking. He would require protection (by the YPG) against kidnapping (by ISIS) and the YPG don't need this kind of distraction. If he stuck only to the "politics" of say facilitiating coalition unity, his contibution may be more useful but he doesn't (as far as I know) have either the necessary language or mediation skills to do this.
As for the illegality of it all, I guess that siding with the "right side" may warrant a token penalty. After all the RAAF is currently dropping bombs on the same people that Matthew is hoping to fight.
