Attic PVC vent piping- hows it done?

Background- my adult son lives in his 6 year old home where roof penetrations are not allowed on front of the house. one upstairs bath's sinks started going glug glug when the toilet was flushed. we found the pvc vent line for that bath and toilet in the attic- it was coming out of the attic floor then going toward the rear of the house horizontally for at least 20 or 25 feet then a 90 degree elbow going up through the roof. The 20/25 ft long pvc pipe was unsuported in places, and had two low spots in it. when he cut the horizontal pipe loose from the roof vent line and raised it he heard water draining back toward the bath- probably rain water - and that water had probably plugged the vent pipe by filling up the two low spots.

Problem is, as he has raised the horizontal pipe to give a good drain back slope, the angle where it connects to the vertical roof line is no longer 90 degrees. How to connect it there? or do you force the horizontal line flat for the last few feet?

He will now need a 45 or 22 1/2 degree fitting in place of the 90 that shoots the vent verticle out to daylight. Might want to get a plumber as there will probably be some pipe measurements that will require some working knowledge of plumbing.

The pitch should be only 1/4" per foot and regular fittings should fit.