Customer can't remove items from cart

Using OpenCart Version 1.5.3.1
One of my customers is complaining about an issue with his cart not refreshing and not being able to remove items from his cart.
So what I gather happened is that he was looking at a previous order, but when he went to return to his current order, the entire item list from the previous order transferred onto his cart for his current order. If he could just delete the items off his cart it wouldn't be a problem, but he's saying things like: the delete button won't work, it won't refresh, he can't set the item quantity to zero.
Any idea what is causing this and how to remedy the problem?

Can you login as that customer through the admin panel and delete items?

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