Some users can't unlink records in interface while others can?

We have an interface built for our finance team to keep track of invoices and which purchase orders they're paid on. (separate tables for invoices and POs). Today one of our team members asked me for help un-linking a PO from an invoice, in order to replace it with a corrected PO record. That's when we discovered that when he hovers over the PO card on the invoice record, he doesn't get the little grey "X" button to unlink the records like I do. We are both editors with full access to the base, and I had him try it on multiple browsers. Any ideas what I'm missing here? Thanks!