Previous home owner had hole drilled for shower drain through the metal web joist in the floor. Is this structurally ok?
Um.. NO. That is a HUGE hole in the width of that strut. and those nail plates are not strong enough to handle compression like that.

They should have added a couple of 2x4s  (Narrow side up) , nailed frequently to the top strut, across and on either side of that join, preferably to the next web, to replace the material lost. 

They should also have fired the plumber or the architect...

It does look like you can add the 2x4s yourself fairly easily, though swinging the hammer on the rear side may be a challenge.