Who's in the right

This is a tough one due to the lack of evidence prior to the collision. I.e. I assume its the windward mark, I assume both boats were sailing upwind not overlapped on port tack and the red boat tacked onto Starboard to pass round the mark.

Potentially two/three rules come into play depending on the prior circumstances which are:

12 ON THE SAME TACK, NOT OVERLAPPEDthis would apply leading up to the mark on port tack
When boats are on the same tack and not overlapped, a boat clear
astern shall keep clear of a boat clear ahead.

13 WHILE TACKING assuming they were both on port and the red boat tacked and completed his tack rule 13 wouldn't apply, if the red boat was deemed not to have completed his tack then rule 13 would apply
After a boat passes head to wind, she shall keep clear of other boats until she is on a close-hauled course. During that time rules 10, 11 and 12 do not apply. If two boats are subject to this rule at the same time, the one on the other’s port side or the one astern shall keep
clear.

16 CHANGING COURSE technically if both boats weren't overlapped then it would be clear ahead and clear astern (rule 12) however if the red boat tacked directly infornt of the black boat rule 16 could apply as it could be deamed as the old 'tacking in my water' scenario and not allowing the port tack boat room to keep clear
16.1 When a right-of-way boat changes course, she shall give the other
boat room to keep clear.

18.2 Giving Mark-Room rule 18.2 (b) could also apply
(a) When boats are overlapped the outside boat shall give the
inside boat mark-room, unless rule 18.2(b) applies.
(b) If boats are overlapped when the first of them reaches the zone,
the outside boat at that moment shall thereafter give the inside
boat mark-room. If a boat is clear ahead when she reaches the
zone, the boat clear astern at that moment shall thereafter give
her mark-room.
(c) When a boat is required to give mark-room by rule 18.2(b), she
shall continue to do so even if later an overlap is broken or a
new overlap begins. However, if the boat entitled to markroom
passes head to wind or leaves the zone, rule 18.2(b)
ceases to apply.
(d) If there is reasonable doubt that a boat obtained or broke an
overlap in time, it shall be presumed that she did not.8

After watching the clip again its a tough one to call, i'd be interested to hear other peoples views and if there was an enquiry what the outcome was