Hi Lynne,
I would like to further confirm a few things on the situation:
1. I notice you only mention one user in your organization has this issue. If possible, would you please further check on some other users?
2. Could you share more details on "Office stays deactivated" during installation? If there are error messages, some relevant screenshots will be much appreciated.
To insert a screenshot, please click Use rich formatting > 
Insert/Edit Media > Browse... > Insert.
3. Please have the user try if he can install and activate the Office on another Mac or Windows PC. We would like to see if the issue is related to the machine or the account.
• This action, if successful, will be counted as one of the five valid installs. Please deactivate the machine after the test.
Generally, to install and activate Office suite included in our valid Office 365, we need to have The latest desktop version of Office assigned to us, which has already been done based on your description. And here are the procedures to install Office for Mac for your references.
Please post back at your convenience and we will follow up on the case.
Best Regards,
Rene