B2B Guests do not want to share O365 account

Hi, we have a B2B customer that does not want to share their user's AzureAD identity.

Since their own employees apparently do not know their own AzureAD accounts, they cannot do SSO to our application.

They also have a problem with the guest's accounts emails changing after they accept(they use email aliases).

The only thing that would work would be using one time email password auth but that is just a fallback and cannot be forced.

Do you have any suggestions?