If you can’t see the troubled user’s free/busy data and you get this error message.

This means that user’s calendar has more than 1000 items in the calendar. You would be asking “Why this limitation?” “Can I increase this limit?”. I don’t think so. I didn’t any information on it yet.

Your solution is to tell the user to delete some items (especially any recurring meetings, if he/she can).

Source: https://support.microsoft.com/en-us/help/2962513/you-can-t-view-free-busy-information-on-another-user-s-calendar-in-exc