The only way to be scored DNF (per the RRS) is to actually not finish the race = he withdrew before finishing. No one can force him to do that. If stipulated in the SIs, he could be scored DNF if he received assistance. If he withdrew after finishing, that would be RAF. Of course, this is assuming nothing weird happened...

On-water judging is most common for Olympics, usually they're looking for kinetics, but they also monitor mark roundings, etc. It's important to note that in fleet racing (especially with lots of fast boats at a gate), it's much harder to position the jury boat to be able to see all points where contact could be made, to definitively confirm overlap, etc. There are other pros and cons to on-water jury.

Mike