hello
@Fluffy pom pom
a warm welcome to DTP
What a worry for you
does your mom in law's GP know about her weight loss; it sounds as though something is not right with her eating .. do let the GP know exactly what the situation is, they may not be able to discuss her health with you but must take note of information from family
it may be that her husband is stressed and simply not taking in the information, or doesn't understand the explanation he's been given, or indeed is so stunned that he's in denial ... maybe he will settle in time ... it would do no harm to let his GP know you are concerned ... maybe the GP could invite him for a 'well man' check up and have a chat with him
you might also contact their Local Authority Adult Services (assuming they are in the UK) and arrange an assessment of your mil's care needs ... a care package might be put in place eg home care visits, day care ...to give them some support with daily living