Yikes.
This is a highly personal question, and it's made even more difficult for us because it isn't even for you - it's for a third party.
What you need to do is sit down with your father and find out what his goals are for this money. You said he doesn't need it right now to live off of, which is good. Find out how much volatility he can stomach (hint - almost everyone says they can stomach more than they really can. it's human nature). Ask whether he's more interested in protecting the principle, or if he's more interested in having it grow over the next 10, 20+ years.
If it sounds like he wants to protect principle and just know the money is there in case he needs it, then you will invest differently than if he wants this to be some sort of legacy/donation/inheritance fund that he'll pass on in 15-25 years.
I know this is a bit of a 'non-answer' but you haven't really given us sufficient info anyone to give you a reasonable answer. Best of luck!