you haven't posted what speed the broadband availability site quotes they should be receiving. if they are receiving below estimates, I would leave well alone and advise them to contact their ISP.
if they are within the estimates, I would probably also leave it well alone. you have no idea what the condition of the line is for that extra 100m, and it could well be that their connection is running as good as it should.
i don't really understand why you would ask advise on a forum, then get so defensive when others give their advise. looks like the majority are trying to give you helpful advise, not just to criticise or patronize you. some comments were unnecessary, but this is the internet, you have to expect that.
you need to weigh up the pros and cons of tinkering with their connection. a small thank you vs being blamed for ruining their connection. I know what I would do.