I don't think the carrier necessarily knows what apps are on your phone. Though I wouldn't rule that out as we learn every few months that one service provider or another has more information about you than they disclose.
If Jennifer's phone was part of a plan with one or more other phones and in that scenario it is possible to be able to view what apps are on other phones that are part of the plan if settings allow that.
But we don't know what they mean by apps. If they mean FB or other things that exist as web sites rather than what are truly mobile only apps then password changes would not require the use of the phone at all unless two-factor authentication was used.
I have no idea how someone would know the passwords were changed unless either someone knew (with certainty) what the passwords were before and those passwords no longer work or if there are e-mails indicating a password change occurred.
This makes things as clear as mud. For me, that means I have discount that changes were made to the phone/apps until a source, such as LE, makes that known.