I don't think having a full-time study partner is the best method to prepare for the GMAT, as such relationships can become one-sided. There's nothing wrong with helping someone, but your time is valuable and you need to worry about your self in this situation. If you have the time, energy, and desire to work with another, then maybe it's okay. But once you start to sacrifice GMAT points for the sake of somebody else's, then there's a problem.
But let's say that you agree to the partnership. Organization and synchronization of time schedules and study plans are crucial. Another important issue is to realize your goals and limits. Don't agree to anything that puts your GMAT score in danger or prevents you from reaching your potential.
I know this sounds a bit cold, but you are the priority here. This is not the time to compromise or sacrifice. I wish both of you the best, but neither of you needs the additional burden of the other. It is certainly possible that a study partner can be beneficial, but your post feels stressed and nervous already. These are not promising signs.
A previous poster mentioned that explaining concepts is a great way to learn the material. I completely agree! And that's why GMATClub is the perfect outlet for that. You can help people when you want and if you want, as well as learn from others with the same amount of freedom. Perhaps you can suggest the website to your friend.