I'm probably a bit late to reply, but... He was @ mentioned in the body of the original post, which Mastodon would have notified him of because mas.to is federating. Opening the notification would bring him to this thread on Mastodon, where everything would appear as it normally does when viewing a thread on Mastodon. From there, you can hold a conversation as normal with notifications and all. Unless you were looking at instance URLs, you probably wouldn't even notice the user you're replying to isn't on Mastodon themselves.