Firstly, I am afraid I have no idea about Scottish Law. However, I looking at it logically, your friend should be in a fairly good position. Since she owned the house before getting married, the most he should be entitled to would be A SHARE of any extra value that the extension made. That said, I am pretty sure that he should be paying maintenance, and if he is not that should be taken into account. (Eg: House originally worth �100,000 (current value). House with extension worth �150,000. Husband should be entitled to �25,000. If he has not been paying any maintenance though, his entitlement would be reduced.
Also note, it would be very rare for a judge to not take into account the above and would want the children to be able to stay in their home if possible. I doubt your friend has much to worry about.
As I also said, she needs to talk to Direct Line (a supervisor or manager) and explain her situation. They are not taking on new mortgage customers but are keeping existing. SHe pay have to pay the standard variable rate as opposed to a discounted rate though.