@CUCOOPE Assuming you are using Microsoft 365 Exchange online for Your emails, and "sometime prompted for MFA" shows that you might have already been setup for Modern Auth for IOS mail app. If there is literally no CA policy getting targeted for this user in any way, you should open a support case to further identify the issue as this will need looking at users information and logs at our backend.
Normally if it was a continuous prompt for MFA, there is a setup issue with the account.
The intermittent MFA pop up , needs investigation of scenario like what actually she was doing and in what order followed by log reviews. Are you sure no CA policy is getting targeted for this user ? Check under What if tool in AAD to confirm, again.
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