As has been said, sea state is the real problem, not wind strength.
Clearly an instructor should not go out to scare the crew - so everyone should be up for it - if just one person doesn't want to go out then that's the verdict for everyone.
Many moons ago I did a YM prep course and exam. On the Monday it was F10 in the Solent - all the candidates said they wanted to go out to get the experience and valued it. We anchored in Osbourne Bay for lunch (where it was a milder F8) and then did MoB practice. Interestingly we learnt that in that strength wind with the main still up, there's no point trying to use the engine - it just can't get enough power into the water and just cavitates whilst fighting the boat's momentum - so it had to be done solely under sail. We were however the only yacht out!
Meanwhile my wife was doing her DS - she voted not to go out into the Solent and so they stayed in Southampton Water (only F9 there!) and then did some theory and pilotage planning in Hythe.
The right choices were made and everyone was happy.