A letter, posted tomorrow, will be there by 1pm (�4.60 postage) or 9am (�10.30 postage) on Thursday if you use Special Delivery. (There are a few places in the country where the 9am service isn't available but most of mainland Britain is covered).
Alternatively, use Yell.com (or the BT directories page) to find a phone number for the Salvation Army in the relevant area. They work to reunite families. While that's normally associated with 'missing persons', you'd probably find that a local SA member would be happy to call round at the house for you.
Of course, if your friend knows where her ex currently works, she may be able to get a message to him via his employer.
Lastly, you could try phoning the local police station. Depending upon how busy they are, you might meet with a refusal to help or you may find that a local community officer would be pleased to call at the house.
Chris