Your local codes may vary, but most all the following for a 2x8" joist:
For a 2x8 joist the max hole is 2 3/8" and max notch is 1 1/4" in outer 1/3 of span, max end notch is 1 7/8". These notches can be on the top or the bottom, but preferred on the top.
This all per minneapolis, MN code:
http://www.ci.minneapolis.mn.us/insp...amemembers.pdf
There are also usually requirments that these notches and holes must have a specified distance between them. Here's another link that'll address some of that:
http://www.ventura.org/vcrma/build_safe/pdf/b-21.pdf
From what you describe, sounds like your plumber did not conform to the standard building codes for your joist size. You should contact your local building code officials since the joist integrity could be compromised. There is a product that may remedy this called the joist reinforcer, though your local inspectors would be the ones to confirm this with. Here's a link that'll show you the details:
http://www.metwood.com/joist_reinfor...ct_details.htm