Is this against the TOS

[Meklore]

According to the TOS:
3.1 Sharing Account Information. You may not share your account or login information (such as your Square Enix ID and password) with anyone except your legal guardian, your minor child, or Square Enix. You are responsible for maintaining the confidentiality of your account and login information, even from your friends and family. If you violate this provision, you may unexpectedly lose access to your account to a third party without recourse against Square Enix. Square Enix will not be obliged to credit your Square Enix Account with Crysta or other points or awards, which may have been used by a third party as a result of the violation of this clause.

So in short, yes.

[Alish]

According to the TOS:
3.1 Sharing Account Information. You may not share your account or login information (such as your Square Enix ID and password) with anyone except your legal guardian, your minor child, or Square Enix. You are responsible for maintaining the confidentiality of your account and login information, even from your friends and family. If you violate this provision, you may unexpectedly lose access to your account to a third party without recourse against Square Enix. Square Enix will not be obliged to credit your Square Enix Account with Crysta or other points or awards, which may have been used by a third party as a result of the violation of this clause.

So in short, yes.

Deleted my post because you beat me to it!

While technically yes, I don't think its a huge problem.

[Hachiko]

According to the TOS:
3.1 Sharing Account Information. You may not share your account or login information (such as your Square Enix ID and password) with anyone except your legal guardian, your minor child, or Square Enix. You are responsible for maintaining the confidentiality of your account and login information, even from your friends and family. If you violate this provision, you may unexpectedly lose access to your account to a third party without recourse against Square Enix. Square Enix will not be obliged to credit your Square Enix Account with Crysta or other points or awards, which may have been used by a third party as a result of the violation of this clause.

So in short, yes.

This only applies if he's giving his nephew his login information, technically. If he were to login and then let his nephew play it wouldn't have anything to do with that section of the ToS.

Also, it doesn't say that Square Enix will take action against the player, it just says that if the player shares their account info Square Enix will not be responsible for account recovery or credits if the player loses access to the account.

So saying it's against the ToS isn't that accurate.