SQL Server: Interplay of personal login and group membership
SQL Server will deny access to a login that has been disabled. This is by design. If it is your intention to allow the person access through the group login, you should simply remove the login that is linked to the Windows account.

SQL Server limits principals' rights such that any rights that have been explicitly denied override rights that have been explicitly or implicitly granted.  For instance, if a Windows Account has a login, and is also a member of an Active Directory group that also has a login, the set of rights granted at connection time actually reflect the rights for both the Account login and the Group login.  If the group login permits access to a specific resource, but the account login denies access to that resource, then access to that resource is denied.

You may be interested in seeing the SQL Server Database Engine Permissions Poster.