Is the money to be earned from additional work at the company halal (permissible in Islam), and is what he did considered a betrayal, given that his friend considered his exclusivity in these tasks a betrayal?

If there is no existing partnership between you, then you are not obligated to inform him or involve him in the works you found in the company. The fact that he participated with you in a previous work does not necessitate involving him in other works, and there is no betrayal in that. However, chivalry and the requirements of companionship may necessitate not concealing the matter from him. It is not forbidden for you to accept these works or what you earn from them.