I have a friend who'll be attending a graduate program in Physics, at a school I would be interested in for when I am applying to grad schools. However, I'm a CS major, so I'd be applying for that.
My question is what can he do that would help at all in making my admission more likely? I know the research he'll be doing is interdisciplinary (Physics and CS) so he'll have some interaction with CS. As a first-year grad student, he won't have much weight, but I'm still curious.
There are a couple of places where your friend may be able to help you. First, some programs have admission committees that include a few graduate students from their own program. Depending on the culture in the program, the student involvement may range from administrative/paperwork assistance to direct evaluation of the applicants. Unfortunately this opportunity is usually not provided to a first year student (or a student outside the department, for that matter). Moreover, if your application is not strong already it's hard for your friend to argue for your case in front of the faculty even if he/she really wants to promote your application.
A second possibility is the interview. During the interview/recruitment weekend, student involvement is quite common. You may be assigned a student host who will walk you around and make sure that you make it to your interviews. During social events (such as dinners, poster sessions, etc) you also have a chance to talk with students in the program. Usually after the recruitment the program will solicit opinions/comments from the students. Thus given the already positive relation between you and your friend, he/she will likely give you favorable reviews after the recruitment, making an admission offer more likely. Unfortunately, this scenario will only help you if you can make it to the interview stage in the first place. But if you've already reached the interview stage, the program most likely will give you an offer anyway, providing that you don't do anything stupid during the interview. So overall the effect of having a friend in the program is minimal in this regard.
To sum up: it's better to work on your GPA, research experience, GRE, etc., rather than wasting time trying to find "shortcuts" like this.