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There may be no good solution to the situation. But there is one possible good solution, involving the simple issue or question:
Look on the front of the safe for a keylock... it may perhaps be under a sliding hatch.
If the safe has a keylock, that is for the 'emergency key' - which opens the safe at a time like this, when the normal means of opening will not work.
Let me know what you find, and what you think. Also let me know whether the safe has a turning dial, or a button/keypad on it.
Drilling it will make it useless.
If your father left no trace nor record of the (probably three-number) combination for the dial lock, and there is no alternate means of opening such as a key-lock, then you have to (since a Safe is a Safe, and not easily opened short of noisily destroying it in any of various ways) conclude that you cannot re-use this, in all likelihood.
A locksmith, if you called one in, would be an expensive way of verifying that, essentially.
You have the most hope of re-use by a careful examination of all records (incl. scraps of paper perhaps, or notebook entries) that your father left.