3rd Party claims can't be entertained

My Aunt who has a policy with CTU, one of her drivers was involved in an accident and didn't inform my aunt. Now that client is a 3rd party claim which now the other insurance is claiming from my aunt and the person who was driving the vehicle at that time unfortunately has passed away to say exactly what happened as he didn't tell my aunt anything. Now CTU is rejecting the claim saying, " they can't entertain it now". Like since when does a 3rd party claim expire? Now you are saying my aunt should pay for all that though she has a policy that also cover 3rd party incidents or claims. What CTU is doing is really unfair here.