You can't on your end.  What may not be clear is that your clients' accounts are not managed by you.  What Inuit Link does is to provide a portal that you can connect your service to your clients' own Intuit accounts (which is the same across all Intuit products) - importantly, by email address.

Once you send an invitation to your client, your client will then sign in to accept that connection to your account.  In the event your client already has an Intuit account, none of the account or security settings in his/her Intuit can or will be altered by what you entered in ProConnect Tax.  If your client lost his/her access to the mechanism for 2FA, such as mobility phone number in your case, your client will need to go through the normal process for recovering his/her account.  In the event your client is not able to recover the account, he/she will need to contact Intuit Support directly.

If all else fail, you can use a new email address and send a new invitation, so that you client can set up a brand new Intuit account.  But your client will not have access to data in his/her other Intuit account through this new account.