In the eyes of the insurance company , they are not being unfair, the same thing used to happen to soldiers going to Lebanon. They don't go on peace keeping missions to Majorca and why should the insurance company carry the risk. However army personnel have compulsory life insurance which covers them in the event of death even if it is in a war zone. In fact that and making a will, is compulsory for all members of the PDF before serving overseas. Also , in the event that he dies due to hostilities , his next of kin will be compensated by the UN, or EU presumably in the case of Chad.
He needs to show all this documentation to the bank and he shouldn't have problems if the amount would cover his morgage.....I think it is 300000. If his morgage is bigger than this he may have trouble, but when he is back from overseas , usually it is not that difficult to get cover, but it won't pay out for death in combat.