Since the rules doesn't specify distances (and to attempt to do so would likely be impractical), deciding if a particular distance was still "safe" would likely fall under the "ordinary practice of seamen" as the Rules (2, 8a) like to fall back upon. What does that mean? I've seen it described as incorporating both courtesy and common sense. I may know that a certain distance is "safe", but it may make the other party uncomfortable and a panicked swing of the tiller may make such a distance quickly "unsafe"; one could thus argue that the courteous distance synonymous to the safe distance.

In any case, this is getting into semantics. Given the other top-level comment, I'm game to delete this thread if you like.

As to fault, details are indeed scarce, and it's indeed likely that the powerboat is in a heap of trouble. My comment is from the point of view that almost no-one emerges blameless in maritime cases. (Some will say that as an absolute, but there have been cases where the apportionment of fault has been 100/0 as opposed to e.g. 50/50 or 90/10.) I doubt this case will make it so far, but if the skipper of the sailboat had held a maritime credential one might expect such scrutiny.