How about keeping the existing setup for joint account data shared with the ex for previous years and having your client create a new account with a brand new email address to which you would link your client's individual engagements going forward?

Having said that, preserving the existing setup is probably not a good idea not the least because that account could be used to initiate new Intuit services without the other spouse's knowledge. 

If they want to preserve the tax documents and correspondence in Intuit Link, they should be advised to download those for their records instead.  After all, clients should be advised against treating Intuit Link as their cloud storage, which it is not designed to be (for various reasons).