Hi heidi1388,
You started from a great resource: Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch
As the issue is only present on one device, you can have them ensure they also clear the website data. The second bullet point in the information below, also founded in the article you started from has the steps.
"Delete history, cache, and cookies
- To clear your history and cookies, go to Settings > Safari, and tap Clear History and Website Data. Clearing your history, cookies, and browsing data from Safari won't change your AutoFill information.

- To clear your cookies and keep your history, go to Settings > Safari > Advanced > Website Data, then tap Remove All Website Data.
- To visit sites without leaving a history, turn Private Browsing on."
As recommended in the same article you can also have them confirm their privacy settings to then clear the history and test. Clear your cache on iPhone. If the issue continues you can also have them test in a different browser.
You can also be sure they have a backup and are using the latest iOS version to test. Update your iPhone or iPad
Cheers.