Hmm.. You might need to check your local codes on wet-venting. IF (and it's a big IF) the toilet & tub actually do tie in to the lavatory, they are both being wet-vented.
I'm assuming toilet line is the one sticking out of the floor and capped (to allow for flange to be at right height on finished floor) and that to the right of it is the lavatory.
but I really don't see how they can merge together. Any idea what direction the sewer line goes? Does it route around or under the tub?
I did a sketch of what I think they should have done (not knowing what it looks like under the floor). Ignore that the red lines go behind the wall-- should be inside/under the wall but it's a rough sketch.
If that large horizontal pipe is carrying sewage, I believe it is a code violation to have the water supply line underneath it and so close. I could be wrong on that-- but if it were my home, I'd have them gut it and have an inspector approve of plans before letting them proceed.
Do you have a photo from the other side so we can see how much of the studs are cut from the other side?
Does this look about right?
View attachment 17192
Also, I'd be concerned about the stability of the wall that has that large pipe running horizontally through the studs. It looks like the holes are rather large and would compromise the integrity of the studs. Too much pressure of someone leaning against them might cause what little is left of the wood to break. It looks like they were trying to avoid running whatever that pipe is underground. I just don't like it.
