Its really pointless to put a hippo in that tank . It will not take long for it to out grow and be unhappy in there. Keeping in mind the tank may be 10 gallons but with sand & rock you dont actually have 10 gallons of water volume in there. That is huge bio load for that small of a tank and its only going to get worse the larger those fish get.
He needs to reconsider and get a small clown or goby. Then further down the road when the tank is better established maybe trying adding one more small fish (by small I mean the mature size is only a couple inches, not how big it is when you buy it)