Since all the water flows through that same pad on it's way through the filter I doubt having it in the tank will be a problem. Unless, that is, he wrung it out in the tank. That might cause a problem.
Not a stupid question, but as another reply stated, you're probably not in any real danger. If you have an all-inclusive test kit you might want to just test ammonia, nitrite, AND nitrate levels so you can see how what your husband did affected the tank chemical balance (if it did at all - my guess is it didn't).
There are such thing as sponge filters which can go directly in the tank itself and act similar to mechanical filtration systems, which is why I say that it probably didn't cause any damage to your system if all he did was plop it in the tank while he worked on the filter.
Most likely it just stirred up some detritus (fishpoop/dead plant particles) and that really won't hurt fish. Fish live in the Mississippi after all! :hihi: A 20-50% water change never hurts though, so when in doubt do that.