on (one's) behalf
For the benefit, interest, or support of someone; as an agent, representative, or in place of someone. Our lawyer will be telephoning the department tomorrow on our behalf to ensure our application goes through as planned. This decision is on your behalf, so don't act so ungrateful!
on (someone's) behalf
1. As the representative of someone else; in lieu of someone. The boss isn't here at the moment, but I'd be happy to sign for the package on her behalf.
2. In the interest or for the benefit of someone else. Children don't understand until they're older that everything their parents do is on their behalf.