The technical process doesn't matter; they'd be entirely within their rights if they only sent your client a "watch token" after the 5/15/30 seconds it takes for you to watch an Ad. You don't owe them anything as long as you're fine getting nothing in return. The only reason they don't do such thing is because they know a large amount of people watch with blocked ads (blocked, as in preventing Google from getting your client to show ads) and doing so would destroy brand reputation and PR.
