The question came up what would be the benefit in having the
instruments communicate with each other using Sea talk or NMEA?

There's also the potential reduction in cabling. You can have multiple
sensors daisy-chained together on the network instead of running wire from
each of them to a display. Less wire, less weight, less to break, less to
pay for. Granted, a single network is a potential single point of failure.
Or a defective device can, in some situations, disrupt the rest of the
network traffic. Fortunately it's easy to plug/unplug such devices to skip
over them. I keep a space seatalk 3 port junction and an extra 25' cable on
board just for such situations.

If he's upgrading the chartplotter make sure the power wiring to it is
correct. The gauge on mine is a little too thin and results in just enough
voltage drop to be a problem when the systems battery starts to drain. When
it was 12v at the battery terminals it was 11.4v at the plotter and got
worse as the system battery drained. Re-wiring with heavier gauge power AND
ground to the plotter eliminated this problem. So have your friend use a
volt meter at the equipment, and then at the battery, to make sure it's
sufficient. It'll save him the debugging headaches later...