It's NO PROBLEM to view their usage history if you have the login credentials.
On the other person's self-serve screen, tap on the "View My Usage" area here:
From there, you can scroll through to see all incoming/outgoing texts and call numbers and durations, as well as MMS messages. Bear in mind, you won't be able to see RCS messaging (i.e. Android chat or Apple iMessage) as those are not through mobile networks.
In order to view how much data has been used, on the self-serve landing page, you see that under the My Data & Add-ons section below. This is quite real-time data so you'll know the allotment and how much they have remaining.
You could also view data used on the usage tab above, however it's only added twice daily there, and represents the data consumed in the preceding 12-hour period:
