If you know the make of the cabinet/safe, then it's probably worth enquiring with the makers. I assume that the deceased husband did have a backup key and although it's empty, he may well have had it secreted somewhere. I'd ask your friend's widow to have a good ratch about, see if she can find it. I suppose the last resort would be to cut it open, but assuming it's a digital one, then perhaps it's worth a bit and so that would be undesirable. It'll probably turn up once the angle grinder has done it's worst!
Again, depending on the potential price of the safe and how much one would cost, perhaps a locksmith could gain entry.