framing / plumbing question

I am working on relocating my kitchen to another part of my house. I mis read the structure of an non load bearing wall (I thought there was going to be a 3x(2x12) header -- yeah I know, why would I think so for a non-load bearing wall......) anyway my original plan was to reframe the wall with 2x6's to provide a channel to get around the header for the 1.5" vent pipe for the kitchen drain
Ok so I find out after some demolition that the 3x(2x12) header is actually a box framed with 2x4's covered with the same 1x12 material that on an adjoining wall is covering the 3x(2x12) beam.
So here is the question. Can a plumber cut a 1.5" hole in the top plate of a nonload bearing (gable end) exterior 2x4 stud wall to run the vent pipe ? As I am doing the framing and my plumber is doing the plumbing and seeing that it is after COB on friday I'd hate to loose the weekend waiting to get the answer from him.
I have several resources on framing construction, but non on plumbing -- if there were any references on the web that would be greate as well.
for reference I live in Maryland
Now the real question is where did I put the receipt for the 25 2x6's that I have to exchange for 25 2x4's.......
TIA Eric
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eric wrote:

#1) The pipe will be just a hair under 2" diam, not 1.5".
#2) Check with the city inspector to see if a mechanical vent ("air-admittance device") would suffice for this single fixture.
Another possibility is the use of a scheme called an "Island vent".
Without getting into the details of these, if the inspector would allow one or the other, you won't have to run any vent pipe.
Jim
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