You will have 24 hours from the time of the notification to review the change. If you don't want the guardian added, you can lock your wallet with an existing guardian at https://security.argent.xyz/lock-wallet if you have default Argent guardian, hardware wallet or Metamask as your wallet guardians.
You can ask your friend/family guardian to lock the wallet for you in their Argent app > Profile > Security > Guardians > Who I protect tab.
When the wallet is locked the new guardian cannot be approved, so will not come into effect.
If the attempt to add a guardian occurred and you're certain nobody else had access to your phone, please contact support@argent.xyz
If the attempt occurred and you no longer have access to your phone, you should recover your wallet on a new phone.