So, anyone can say that you are lucky guy! The answer is simple - just hold so long as you can. Sure i you can invest ETH to another alts, but I can recommend to hold Ethereum as most liquid coin.
Can not hold 100% of ETH at this time. If it was me, I would advise him to sell half the amount of ETH that he is holding to buy Altcoin at this time. I think Altcoin will make a lot of money when the market recovers