If BoSI made an offer and your friend accepted it, then it's valid. If they wrote to her to confirm, then she is absolutely fine.
However, if BoSI offered your friend this agreement and she did not accept it, then I would say that it is not valid. So if the confirmation asked her to sign it and return it, but she did not do so, then I would say that she has no agreement.
If it's a Buy to Let, they might appoint a Receiver and your friend would need to go to court to get the Receiver removed.
If they classify your friend as being in arrears, they must complain immediately and go to the Ombudsman if it's not resolved.
Brendan