I think it's most probably allowed according the law. There isn't an air starter unit on every airport in Europe, and if you cannot start with your own bleed (bleed or APU problem) you have to do it with a running engine.
I remember when they did it:
Park the aircraft, engine 1 off, pax disembark, start eng 1 with eng 2, eng 2 off, start fuelling, catering aso, start eng 2 with eng 1, eng 1 off, let the people in, start eng 1 with eng 2. Off you go!
One airline would limit itself by quite a bit if it wouldn't allow this procedure.