Lenders always work with "what if?" scenarios. I could see a mortgage manager (or, more likely, the computer algorithm that he/she uses to determine whether a loan can be advanced) thinking along the lines of "What if the lady who's moving in with this couple were to die? How would they maintain their increased mortgage payments then?"
Further, as you wouldn't own a share of the property which formed the security for the loan, the lender would probably regard your £400 per month as, in effect, 'paying rent'. Lenders often have different rules (and frequently higher interest rates) when mortgaged properties are being used to obtain a rental income.
So, my own feeling is (regrettably) that the lender wouldn't be prepared to 'play ball' with your suggestion. However it might still be worth your daughter and son-in-law making a tentative enquiry to the lender.