Seen to much of this. But why does a customer always wait until job is complete or as near as, then chuck them off the job.
Always check. In some cases they seem to be embarrassed and don't want to be seen to be looking over the workers shoulders. Simple solution, bring up a few cuppas and have a gander. If you don't like what you see, raise your concerns straight away.
You may be spun a fib that when grouted etc it will be fine. It won't.
If you can't raise it then, wait until he/she goes home on the first day. Get up and go through the job throughly and if it's bad and we all know what is and isn't whether your an office worker or tradesperson, you will have seen many examples of good and bad tiling ie pub/restaurant toilets, shopping centres and friends homes etc.
If it's bad get them out asap as it will be quicker and cheaper to rectify.
If in doubt talk it out, if still in doubt chuck em out.