I hope the pipe didn't terminate in the wall, but continued up to the roof and outside. Can't think of any other reason for the odor. You're using the sink and the trap has water in it ???

The gas is escaping the wall through the holes where the pipes are. It could be coming from the piece he added or the previous drain pipe. I am going to guess and say it is the old drain pipe that did not get capped. There is a much larger volume of supply gas.
I wonder why he didnt tap into the existing roof vent?

"It seemed odd to me to vent into an interior wall but the plumber said it was code."

Did he put an air admittance valve on the end of the pipe?

A plain open pipe is not allowed.

I was talking to a plumber today and he said the above is OK by code, but there is supposed to be an access panel to get the end of the pipe. Hope you have that so you can add the valve to the pipe if you need to.

Thanks for all the replies. The plumber did put an air admittance valve on the end of the pipe but didn't say anything about putting in an access panel. The previous drain pipe didn't have a vent. I'll have to put in access panel and have the plumber come back to make sure vent is working correctly. Thanks