The drain line from the dishwasher is fine but I see three code violations on the sink & disposal drain lines.

1. The two drains are connected by means of astandard sanitary tee. Code requires a baffled tee.

2. The P-trap is too low, which resulted in installing an 1-1/2" vertical offset on the fixture arm. Code requires a 1/4" per foot downward pitch from the trap weir to the vent.

3. No cleanout. Code requires a cleanout on the upstream end of all drain lines. If you have a removable trap removing the trap will serve as the cleanout but when the trap is glued in you must install a cleanout on the line.

I can certainly understand why he installed the unit in that manner, but there is a much simpler solution.

First off, get a "disposal type" end waste or center waste kit. When installing a disposal type kit you begin by discarding the 1/4 bend (90 deg elbow) that is supplied with the disposal unit. The disposal type waste kit will have a cross tube that has a flat flange on the end. That tube is then installed on the disposal in the same manner as the 1/4 bend, but the line will no come away at the same elevation as the disposal discharge port. You then shorten the length of the tailpiece on the opposite sink and connect the two lines together with the "Baffled Tee" that is supplied in the disposal waste kit. In this configuration the tee is higher permitting the P-trap to remain at the correct height to provide the required pitch to the fixture drain line.

In the configuration in the photos it would probably be best to install a disposal type center waste kit. This would then put the tee midway between the two sinks.

You would then have to cut the fixture arm near the wall and install an 1/8th bend (45deg sanitary elbow) and extend the fixture arm out towards the center where it will meet the P-trap tail piece.

It is important that you extend the fixture arm from the wall to reach the trap. Code prohibits adding an extension tailpiece on the P-trap.

I have prepared an illustration that will show you how the disposal type kits are configured.

From your pictures, the new P-trap goes at the bottom and extends to the wall?

Yes, the tailpiece from the tee can be cut to the appropriate length to fit the input of a P-trap