Where to draw the line depends on your comfort level
If they're getting too loud and rambunctious for your liking, then just tell them to settle down a bit (just like kids LOL). If they don't, then you can separate them. As long as the limit you set is 'reasonable' then they'll soon learn where the limit is, and that they need to settle down or playtime will end.
Boxers are naturally loud and enthusiastic players - but you're quite right that many other dogs won't see it that way. How old is your boy btw? If he's just a baby puppy, then I'd be inclined to leave it to the older dog to teach him when enough is enough and how to play nicely (bear in mind that babies get all manner of leeway - she'd start to teach him manners as he gets older). If he's not just a baby though, then I don't think there is anything wrong with tempering their play a little.