Alternative broadband set up

[#1]

I've been a Plusnet customer for several years and am very happy, I do have a question though, my son takes his laptop to his grandmothers most weekends and the summer holidays, which are coming up, the problem is my home broadband signal won't reach to her house (its only 200 yards as the crow flies), so what is the best and cheapest option for me to get him a connection over there? Mobile broadband won't work as I've tried to help a neighbour over there with his set-up.
Any ideas?
Cheers
Alan

[#2]

Probably nothing "legal" if you're thinking of extending your service. 
Do you have line-of-sight between the 2 properties?
If so there are solutions, but probably not at a price you'd be prepared to pay. 

[#3]

The best solution would be to use broadband at her house.
You could sign her up to the 'Value' package, and if there isn't enough included bandwidth, you could tell the lad to also take the router across to use (ie, perhaps the simplest way to ensure that although he is at hers, he would be logging on with the credentials for your package stored in your router, and therefore using the bandwidth included with your account.)
Nothing particularly dodgy about this, although if either of the lines are 'borderline' you would need to be careful about telling the router to disconnect before unplugging it.