Connection of ESD return path the System GND
You have an ABS enclosure and some serial ports those need to be protected against ESD.

I think your review engineer is right. There is no need to the capacitor C1 and resistor R1. 

First of all I recommend every body to reread "System level ESD co-design" That is Published by John Wiley and Sons.
ESD is acting like a 300+ MHz signal. your ESD protection circuit and its PCB must be able to pass this signal through itself. It is true and also a good practice that the return path of the ESD towards the main ground pin of the power supply connector must be some how separated (physically or by moat). If the protector device inject the ESD into a ground plane, it could make phenomena line soft errors.

Therefore, It is a good practice to have a ground polygons under all of the connectors (all the connector body must be over the ground polygon and the polygon must be bigger than connector occupied space on the board) and connect the protection devices to that polygon and through it to the main ground pin of the power supply. The injected ESD to this polygon by protection device will drained into the main ground pin of the board (with shortest possible way) and will not bounce back to the sensitive parts of the circuit. Obviously, the protection devices must be close to connectors too. your ground plane/planes (if you have multi layer board) also connected to the main ground pin of the power supply connector but the ESD will not make trouble for them because drainage capability of your main ground pin.

In case of ESD injection onto plastic/metallic connectors the ground polygon that is under them in the assembled layer, will absorb and guide it towards the main ground pin.

in case of ESD injection onto ABS enclosure, you must be worry about any ABS pipes or gussets those are connected to or close to your PCB. These unwanted proximity with enclosure could guide the ESD behind your protection lines and make bad damages for you. All such proximity with enclosure must be protected by direct guard ring/trace towards the protection polygon that has direct adjacency with connectors. 

In case of ESD injection into high frequency signal cables, all of related pins have been protected by proper TVS and related Y-shaped connection on the PCB. the Y connection will guide the ESD towards main ground pin through the ground polygon and the protection device has very few attenuation on the main regular signal. 

In case of ESD injection into low frequency signal cables, all related pins normally protected with ESD capacitors or special protection chips and on the PCB the thick Y connection is recommended to use at the entrance of the protection device and narrow trace 90 degree connection recommended for after protection device towards protected circuit to maximize the impedance discontinuity and reflection of unwanted high frequency waves those are injected towards sensitive parts.
 obviously ESD capacitors with footprint 0603 or lower could be destroyed by passing through 15kV ESD. 
Please look at the following presentation by Bosch. http://www.emcsociety.org/2011%20Events/IEEESEMMLCCESD.pdf

Normally I use the combination of a Spark gap or intrinsically spark gap (the grounded polygon adjacent to connectors at their assembled layer (top or bottom)) and the protection device that is TVS, protection chip or ESD capacitor.