Plumbers don't install vents????

I'm helping a buddy of mine build a new house. We bid out the various trades, etc and hired a plumber to do the rough-in, top out, yard line, under slab, etc.

The plumber has completed the rough-in with the exception of the roof vents. He ran sch40 PVC to within a few feet of the roof decking and won't complete the job. He says he doesn't work on 14/12 roof pitches and refuses to finish the job.

Unfortunately, the home owner paid him for the rough-in already and now doesn't know how to get the vents installed.

Is this common practice in the plumbing industry?

Whose responsibility *is* it?

I have never had a plumber back out like this. Thoughts?

Thanks,

David