can I plug my laptop ADSL modem into his phone line, configure a network connection using my ISP details, and use my broadband signon?
No
Or is there a link between my home phone number and my ISP details that would prevent this? Yes
Think about it - it;'d be a massive scam - just one person needs an account and the rest of the UK could use their details..
Get yr friend an ADSL wireless router - change out of £30.