As the OP has described him as a fitter and has obviously been paid to install that, then he should be a professional and not an enthusiastic DIY'er who has tried to do things correctly IMO.
If he has been employed by the OP as a handyman type person then fair enough and I apologise for being so critical. If he hasn't and was employed as a proper fitter then that's not a very professional job at all and why our trade can get such a bad rep sometimes.
Just because he may have used some correct fittings and installed drain off's, which I'm sorry but just aren't needed in that situation, doesn't make it OK IMO. That pipework should have been much tidier, tight to the wall and clipped, flexi's too long, no need to use push fit, make good the wall if he has created those holes etc.
didnt you know: the more fittings you use the less chance of leaks