Domain Mapping: Forced to Log Back In

I have a network of 900+ users. Whats strange is only 1 user is complaining that they must relogin after visiting their frontend coming from their wp-admin, and likewise.

Ive logged in as the user and tried to replicate the issue on many occasions, but unsuccessful.

Ive had the user clear their cookies and cache, but they claim the issue is also happening on tablets, phones and pc too. All those are on separate devices and the user even states they use different browsers.

Do you have any idea how can I correct an invisible problem? I just cant seem to track it down, yet the user in constantly sending in support requests stating they must keep logging back in.

What would you tell them?