John Gill Commentary 2 John 1:11

"for he that giveth him greeting partaketh in his evil works." — 2 John 1:11 (ASV)

For he that biddeth him God speed
Wishes him well, and success in his ministry, or in a friendly and familiar way converses with him:

is partaker of his evil deeds ;
he has fellowship with him, instead of reproving or shunning him, as he ought; he is an abettor of him in his principles, and so far joins in the propagation of them, and helps to spread them, and gives too much reason to think he is one with him in them.