My wife works for a pet control company so I can say this for sure. The chemicals used are most dangerous during the first few hours that is why you cannot stay in the treated area.
Most of the chemicals used are water based so it will kill an aquarium off with ease. It will also kill birds if not sealed off correctly.
When I say sealed, I mean air tight sealed as the fumes from the spray will travel and end up in the tank. Just covering the tank is not good enough, The technician should have explain that of you friend should have asked about his tank's safety before allowing them to enter his apartment.
Any time the Exterminators come to my place I have duct tape and a large plastic tarp that they used for painting that I use. I seal off from top to bottom and all sides that it's air tight.
I've been doing this for a few years not with success.
Also for a side note, the corner seats on the trains are where bed bugs are found in the subway.