I can only address this as a person who has traveled internationally for fun, never for work. However, being vegan, I try to take as much of my own food as I can when possible. As you/he probably know, what you're allowed to take varies a huge amount by country, and I agree that frozen home-cooked meals are probably going to be a no-go in this scenario for most countries!
I'm not sure if you are referring to buying pre-packaged food that isn't canned in the US prior to leaving or if you're talking about trying to find it at the destination. I'd be worried about packages that aren't something like a can leaking in luggage, and they'd be heavy! I probably wouldn't rely on being able to find pre-packaged food at grocery stores everywhere, but depending on the destination that might work out.
If he has access to hot water (coffee pot or microwave in a hotel room, probably?), he might consider dehydrated foods. He can either get entire meals like what backpackers use (expensive but probably cheaper than a restaurant), or he can do something like take oats, dehydrated fruit, and nuts with him so he can make a bowl of oatmeal for breakfast, or dehydrated veggies and beans for lunch, etc. (Not just dry beans from the grocery store, as those take forever to cook, but you can buy previously cooked and then dehydrated beans, which are quick to rehydrate in hot water.)
http://www.harmonyhousefoods.com/ is a great source for entire meals (mostly soups, I think) or individual vegetables or fruits or beans that you can then combine on your own.
If you get really into the dehydrated food thing, you can make your own meals at home, dehydrate them, and take them with you. How DIY you go will probably depend on the planned destination. Some places really want things to be in a commercial package, not in your own Ziplock bag from home. So far, I haven't been to any countries that have had a problem with any dehydrated foods, even those I made myself. I've actually been surprised by what IS allowed. I was even allowed to take nuts to the Galapagos islands, where they're super strict.
I've also taken homemade kale chips, homemade "energy balls," and homemade granola bars to 3 continents without any problems. I'll just eat stuff like that as a meal if there's no better option.
I do make use of local grocery stores or outdoor markets. I've had some great salads, cheese and crackers/bread (back when I ate cheese), peanut butter and jelly sandwiches, fresh fruit, etc. For meat eaters, you could add something like salami or some other meat that's already cooked.
And then there's street food, of course! Obviously that will depend on the location, too.
I'm curious to see what others will say, as I'm always looking for better ways to keep up my healthy diet while traveling!