Excel freezes when a certain person is in the same file

We have three people (computers) accessing our onedrive files. All seems to work fine except for Excel and only for one person.

When A & B are in the same Excel file everything works fine.  When C opens a file that either or both A & B are already in or if A and/or B accesses a file that C is already in C's Excel freezes up and just spins until the task is ended or computer is restarted.