There is NO SET RULE, it is up to each insurer, so look in your policy.
Whatever you will have to declare it, but of course if there is no claim they cannot witdraw any NO CLAIM bonus, that does NOT stop them loading on the renrewal though.
It is TOTALLY LEGAL to settle without insurers, involvment but they must be told about accident, as other wise withholding info could invalidate.
Nothing you do has ANY bearing on any future claims he may submit to yourself or the insureres over the accident, so even if you pay him out he can still come back and WILL be successful, as your payment will be an admission of liability so will be easier for him.
tabetha