I really need your help with communication with the manager concerning a coworker.

I really need your help with communication with the manager concerning a coworker. I work for a small organization, with a director, two managers, a coworker (we report to the same manager), and myself (we’re in the process of hiring an additional person to support one of the managers). We’ll say that I work as the administrative assistant to the organization, which means I assist everyone to a certain extent. I report directly to the director, and one of the managers, and assist the other. The coworker works for a different section of the organization, to whom I assist as well. They tend to ask for my help at the last minute. I won’t go into it, but it seems like they’re procrastinating. They asked for my help today, with something they want to be done…today. The only reason they asked for my help is that someone from the helpdesk is on vacation until Tuesday. When I explained to them that my meeting with our manager is tomorrow and I tend to receive more projects then; they tell me that our manager told them that the projects I have for them are coming to an end, so I might be able to lend a hand. I said “that’s odd…since the manager just told me that I’ll be working on a project very soon. Also, I’m supporting the other manager until she hires someone for that position.” The response I was given was “Oh, they did mention that too.” My face… -__-

I want to communicate with my manager, that my coworker should discuss if/when they’ll need my assistance, with management first. Then allow management to communicate this to me. I don’t want the coworker to get in the habit of asking for my help, then treating me like I’m their personal assistant.