there is no bio filtration (live rock or bio media for nitrifying bacteria)? if not, any life added will cause ammonia that can't cycle. he needs to get some bio filtration in there or do big (75%+) water changes daily.
eels are messier/bigger eaters than most fish but nothing should cause an ammonia spike if the nitrifying bacterial filter is adequate in the tank...
if he's got bio filtration, then maybe it didn't fully cycle...or if it cycled but tank is tiny and huge bioload added all at once, it could have a minor re-cycle...