I also live locally, and your blog doesn't give any idea why this company is incompetant? Its fairly reasonable for a part not to be carried around with them all the time and have to be ordered in. For all we know the incident could have happened on a Thursday, called the company out Friday (order then placed), part arrives Monday put on Tuesday morning. Sounds fairly reasonable to me!?
There are lots of houses that don't have central heating, and other forms of heating could have been used in the interim - or, of course, as she's your friend - surely they could have stayed at your gaff or another friend/ relative for the short period?