If they are using the standard text message app on their phone, the text activity is still visible on the text log in your on-line My Verizon account even if the texts have been deleted from the phone. If they have an iPhone and are using iMessages (or using a third-party text app on their phone, regardless of which phone they're using), the text activity won't show up in the text logs since those texts don't pass through Verizon's servers.
