How much will I receive if they only files for one benefit?
If your spouses birthday is Jan.2, 1954 or later the option to take only one benefit at full retirement age no longer exists. read they will be effectively filing for all retirement or spousal benefits .
Will this decrease my spouses retirement benefit?
Benefits paid to you as a spouse will not decrease your spouse s retirement benefit.  In fact ,the value of the benefits you may receive , added to their benefits , may help your spouse decide if taking benefits sooner may be more advantageous .