This would make a great exam question orribleal. It's the sort of thing that used to keep me awake at night when I was a student!
Assuming that the house was held as a joint tenancy, this charge can only be enforced against your partner's mother if she gave her informed consent to it. There are various legal presumptions regarding undue influence and what constitutes informed consent. The good news is that it is usually down to the Bank to prove that informed consent was given. So if your partner's mother didn't sign, didn't know what she was signing, of didn't know the effect of it, the Bank will have a hard time enforcing.
Even if the charge is enforceable it might not be in the Bank's interest to sell the house. This is a second legal charge and is subject to the rights of the first legal charge holder (the mortgage company). Is there enough equity in the house to repay the mortgage plus �170,000?
You need to get formal legal advice from a solicitor / CAB / Legal Advice Centre. You should be able to get free advice if on benefit. Good luck.
LS