Need some help
 

My mates classic broke down today and the tentioner snapped off that run the cambelt does that mean the engine it f**ked or can we just put a new tentioner on ?

Very likely that it's going to be ****ed I'm afraid, be extremely lucky if it's not

As above :(

At the worst it will have done 16 valves & some piston damage.

Yep Valves almost definite and piston fairly likely .

has it actually jumped the belt or simply snapped the tensioner??

get the covers off, align everything (refit belt if required) and do a compression test, this will give you a good guide as to damage

i did have a car years ago that did similiar after a ham-fisted mechanic used a scaffold pole or similiar to tighten the tensioner on!! customer stopped the car on noticing a strange noise and was very lucky to find that after some reapirs it was perfect!!