Tell your friend to call PlusNet and ask them to investigate, they'll ask her to do some diagnostics, if they don't find anything on the internal wiring during the diagnostics they'll send out a BT Engineer to check the line between her and the exchange. No point putting up with a crap service, it's best to get to the bottom of it and I'll guarantee it won't be PlusNet's network at fault (either internal wiring or the BT line will be at fault, at worst it might be the PlusNet provided router if it's old). Generally the PlusNet service is excellent.
Use the link below, it will give you an indication as to what speeds you can get and the other broadband types in your area.
http://www.samknows.com/broadband/broadband_checker If it says you can get fibre to the cabinet (FTTC) your best bet is to fill in your details on an ISP's website (
such as this)to check if it's available in your street, Samknows doesn't have that info, just the info about the exchange itself and if it is FTTC enabled.
HTH!
