A way to emulate MDI on newer versions of Excel?

Odd question here, I hope this is the place to ask. I am supporting a client that still uses office 2010 😕 It is a long road, but we are trying to slowly get them away from it. The main reason for not wanting to move away is that Excel 2010 still supported the MDI (Multiple Document Interface), while newer versions switched to SDI (Single Document Interface). This client is hellbent on the idea that his 20+ year old model breaks with anything that doesn't support MDI, which I know isn't the case, but anyways. For the time being we are looking for a compromise as part of our strategy to eventually get him away from using 2010 ( huge security issue come October).

Are there 3rd party add-ons or using VBA to emulate the MDI in newer versions of Office? We have looked at ExtendedOffice: namely this: https://www.extendoffice.com/product/office-tab.html but we don't know of anyone who has used this before, plus as a cybersecurity industry we are skeptical of anything being created and maintained in China.. Any help would greatly be appreciated..