The court could only order that she makes best efforts to release him from the mortgage. The lender is not party to proceedings.
If the lender really has told him she hasn't contacted them that is a breach of GDPR. They certainly cannot tell him if she has applied and been refused. Indeed, they may not class her booking an appointment with a mortgage adviser and being told how big a mortgage she can get as contacting them about this mortgage. There is, of course, also the possibility that she has contacted other lenders or used the current lender's online calculator to check affordability. Her salary is not the only factor determining affordability. If she has other loans or significant outgoings she may not meet affordability criteria.
If your husband agreed that she could keep the house on condition that she make best efforts to remove him from the mortgage, it is unlikely the courts would interfere with that. Even if he hadn't agreed, the court would have wanted to ensure that she had a roof over her head so, if they had been aware that she couldn't get a big enough mortgage to buy a house with whatever deposit she had available, that would have been even more reason for her to keep the house.
Since your husband knew she was a nurse, he should have been aware that she would not be able to release him from the mortgage at the time they divorced. There is no way she would have been given a mortgage of that size on a nurse's salary. Indeed, given that both sides will have made full financial disclosure, I'm sure the judge was aware that she wasn't going to be able to release your husband from the mortgage any time soon.
If she did not make a full disclosure or she lied about her income or assets it may be possible to re-open the financial settlement. However, provided she made a full and correct disclosure, there are no grounds for changing it. If you want to know for sure, your husband needs to consult a solicitor, but on the information posted I don't think you will get anywhere with this.