It seems like you've already taken several troubleshooting steps to address the issue. Since you mentioned that it only occurs for a specific user, it's possible that there might be some user-specific settings causing this behavior. Here are a few additional suggestions you can try:
Check for add-ins: Ask the user to disable any add-ins or plugins they might have installed in Outlook. Sometimes, third-party add-ins can interfere with the normal functioning of the application.
Create a new Outlook profile: Sometimes, issues can be related to the user's Outlook profile. Try creating a new profile for the affected user and see if the issue persists. To create a new profile, go to the Control Panel, search for "Mail," and click on "Mail (32-bit)" or "Mail (Microsoft Outlook)." Then, click on "Show Profiles" and follow the prompts to create a new profile.
Repair Office installation: Use the Microsoft Office Repair feature to repair the Office installation on the affected user's computer. This can help resolve any issues with Outlook or its associated components. You can find the repair option by going to the Control Panel, selecting "Programs and Features" (or "Add or Remove Programs"), and selecting Microsoft Office. Then, choose the "Change" option and select "Repair."
Temporary workaround: While investigating the issue further, you can suggest a temporary workaround to the user. Ask her to paste the link directly into the email body without using the link button. After pasting, she can select the link text and use the "Hyperlink" option in the ribbon to add a hyperlink to it manually.
If none of these steps resolve the issue, it might be necessary to involve the IT support team or Microsoft support for further assistance. They can provide more targeted troubleshooting based on the specific environment and configuration in use.