He put freshwater bacteria via a pump sponge into his saltwater tank to 'help' it out...
I hope you have a good skimmer. Get the sponge out. The bacteria are now dying, and depending on how many, could create a fairly large dead biomass in your saltwater tank. Your saltwater bacteria will not die as a result of this: they will consume the remains of your freshwater bugs, but may not be numerous enough to do it all. You may have provoked a small cycle, much as if you had tossed a dead fish into your saltwater tank to let it rot.
Take the following steps:
1. run carbon---this will help remove resulting ammonia.
2. test for ammonia/nitrate/nitrite; you know---the strips.
3. dose very cautiously with Amquel if you can't get the ammonia down with carbon. Ammonia is a killer.
4. tune your skimmer to run wet and keep dumping.
5. do a 20% water change.
As a rule things from freshwater that go into saltwater die within hours: with fish, it's kidney failure: memory eludes me as to what it is with a bug or bacterium, but it's something like what happens to shrimp that are put into a different salinity water: osmotic shock---their insides and outsides aren't at the same pressure, and they can't regulate it because shrimp can't sweat through their shells.
Don't mix the two environments: it's not even a good idea to trade dry nets from one to the other. And I hope like everything you've never used copper in that freshwater tank you borrowed the sponges from! THat would be a major, tank-contaminating, permanent disaster for the saltwater tank.