Dave
Glad to have you back, and am hoping you have made a full recovery from the heart attack.
In any event, I’m not educated enough to give you the explanation you are looking for, except to say that a conventional digital voltmeter won’t work, but I know they make affordable adapters that can be used with a conventional voltmeter.
But, I can speak about doing the basic stuff first with these systems before getting buried in output tests.
Your cousin’s 25hp has no spark on either cylinder, so the first basic test you would want to perform is the stop circuit elimination test. Pressing the stop button grounds the powerpack’s internal capacitor keeping it from holding a charge. Look for the two black stop switch leads coming from the engine pan. One lead will be grounded, the other stop switch lead should end up in the five pin rubber connector on top of the cylinder head. The tricky part is pushing the black stop switch lead pin out of the rubber connector, the amphenol pin tools make this easier, but I’m sure you can rig something up. Split the connector and dip the stop switch side of the connector is some rubbing alcohol to loosen the rubber’s grip on the pin. Push the stop switch pin out of the connector, then reconnect the rubber connectors and check for spark. Please do not cut the leads, and simply disconnecting the stop switch’s ground lead is not a complete elimination test. If the engine has spark with the stop circuit pin removed then you will need to look for shorted stop switch leads and/or replace the stop switch.
The next simple test is to simply look at those amphenol rubber connections, they do cause problems. Sometimes the pins/sockets get pushed back in the rubber connectors, the pins get bent/misaligned also. Gently tug on each lead in the rubber connector to make sure the wires haven’t broke free from the pins/sockets. Always use rubbing alcohol to "lubricate" the connectors to make reconnecting them easier. Never use WD40/grease, or anything else that might actually conduct electricity.
Your cousin’s engine has the primary push on coil connectors coming from the powerpack also, these connections get loose, and corroded, especially in salt water. Pull the primary connections off, check for rust/corrosion, and clean the terminals as best you can.
Then, crimp the primary connectors slightly so they will fit tightly on the coil primary studs. Go ahead and use a little grease/moly lube on the primary coil studs to help prevent further rust and corrosion. You might want to check the coil secondary connections/wires also, those spring pins get rusted and loose continuity.
Finally, check the powerpack and coil grounds with an ohm meter set on the low ohm’s scale. Checking between each coil’s ground strap
and a good engine ground should show as close to zero ohms as possible. Do the same test between the powerpack’s ground lead and a good engine ground.
Once these tests have been completed, we can move on to resistance and output tests if there is still no spark. I would be glad to send you one of my PRVs, but I am guessing you want to get the problem solved ASAP.

The CD ignitions need a peak volt meter, not one for the spark plug end. The most common we used in the 1980’s (when I was servicing OMC outboards) was a Stevens CD-77. It seems to be used to this day too.

Here is one example available for a reasonable cost:

http://www.ebay.com/itm/Digital-Multime … 43&vxp=mtr

I may be, and frequently am, wrong; but my Fluke DVM has a min-max feature. I never tried it as a peak reading volt meter but I would expect that it would read just that: The ONE peak in a long sample, as it does for conventional AC and DC readings. The meter above is nothing special but the plug-in adapter that is show with it allows you to monitor the peaks of the waves continuously.