It looks like it's close to a wall, so unlikely to be walked over or subject to a heavy load. Based on that I'd leave it alone.
Your other option is to glue and screw another joist to it - although it would only be able to sit under the pipes, but might still add strength. From the pic it looks like you've only cut a small hole in the floorboards so maybe not an option as it won't feed through.
Only other option is a bit of steel strip, say 100mm (or whatever space you have bet underside of pipe and bot of joist) x 6mm x 500mm long- it would slide in flat turn upright once in ceiling void.....and screw on.
Is it near the middle of the joist or close to an end?
I'm guessing the plumber cut the notch after the electrician....wot a muppet