take all the remot controls with you when you leave, that really pisses them off

Good one!
heard of prawns in the hems of suits too!

ok if were getting childish

washing up liquid in washing machine.

turn off the ac, throw some prawns as far into the vents as you can, then put a load of talcum powder in for good measure....(note do NOT turn ac back on)

do the same with his car ac if you have access

pour sugar down the back off everything and into everything. you will have an ant farm in no time, a TV with ants coming out will not amuse him

if its a villa leave a load of cat food outside, turn the place into the local strays hangout

register all his email addresses with every site you can find online, viagra, dating, credit cards etc you name it. wont be long until he has to abandon them and theres a good chance he'll get stung with a virus. also feel free to post his credit card details online although that could well come back to sting you

or go here
http://www.thepayback.com/

scratch all his CD's and then put them back in their holders

Place a very strong magnet next to hard drive or laptop

On the information you provide,I am certain if your friends wish to divorce,
then they could raise an action a UK Court.