While most cars require 12 volts, the amperage output only matters if you are also planning to use it for jumpstarting, In other words, for a regular charger, nearly any 12V charger will handle a V8 since all it’s doing is charging the battery, and the battery in a car with a V-8 really isn’t any bigger than what I have in my old Corolla.
Now, if you need one that can do jumpstarting, then you need something a bit heftier than my little old ‘yota would.
Also, “portable battery charger” is a bit vague, simply because some people use that term for something like this, a charger about the size of a toobox as opposed to a cart charger while some use it to describe dedicated jumpstarters.
I would wager that what you are looking for is probably the last one. Personally, I have not had luck with any of the small ones even with a little four-banger, so your man would likely need something at least 750 peak amps for a V8 (the one I linked is 950), and it’s better to spend a little more and get one that’s a little overkill than save a little and get something that isn’t up to the task.
It’s a bit late, so I have to cut this short. If that is along the lines of what you need though, I can help you out more tomorrow.