I hope no one minds a question from a lurker who drops by from time to
time.

The subject of experiencing a knockdown was recently raised and I'd
like to further extend the subject by asking what can/should one do
when knocked down?

I was recently on a 21 footer with my son-in-law at the till cruising
along fairly well with 25-30 knot winds when he noticed we were
heading into an area of additional wind. While alerting me, he
disengaged the mainsheet and when we rolled he let it fly. The roll
took us over nearly 90 degrees and we shipped water over the coaming 6
to 10 inches in depth. Realizing how little effect releasing the main
had had, he tried to uncleat the jib. This is where we found that
despite the angle of the sail to the wind, little or no wind was being
dumped and it was a difficult task, to say the least, to uncleat the
sheet. In our case, he tried three or four mighty tugs while on the
high side of the boat with no success, so he dropped down to the low
side in order to grasp the sheet close to the camcleat and after a
couple more mighty tugs it came loose and we righted.

Did he do the right thing? Or should something else have been done?

We remind that it was a 21 footer, with a centerboard.

Thanks.