Sponge filters can be ok if used properly and not overpowering, but I wouldn't I suggest that be the only filtration. Your friend would do best to add a canister filter, hang on filter, etc.
Part of what will determine what is enough filtration is size of the tank, animals within (what and how many), feeding habits, and how often maintenance is done. To run a healthy saltwater tank with only a sponge/powerhead combo, water changes would have to be frequent and feedings not overwhelming. Lots of live rock and live sand, very few animals... the idea is to keep the waste level in proportion to what the natural biological filtratin can handle and and to help it with lots of small water changes regularly.