In the truest sense of the word, a codependent relationship usually involves a substance abuser and someone who tries to cover for them and/or tries to control their substance use. In less extreme settings, you will see someone who is very self-confident with someone who does not feel the same way about themselves, and so they rely on the confident person for substance and meaning. What you will usually see is the codependent person being very clingy with the other. Their friendship gives their lives so much meaning that they want to be around the other person all the time. When the non-codependent person isn't around or is with other friends, the codependent person feels miserable, left out, and maybe even betrayed. They may become either sullen around their friend, or voice their discontent in a very extreme way.
The first step for the codependent person is to recognize they are being codependent or, at the very least, recognize that their life is miserable when they are not with their friend. They would benefit from stepping back a bit from the relationship and developing their own circle of friends (not an alternate one-on-one friendship just yet, as they may just repeat the pattern of codependency, albeit with a different person). They would benefit from discovering what different things would make them happy, such as hobbies or community activities. Anything that involves a group of people would be most beneficial. Most of all, the person needs to avoid elevating anyone to a higher level than themselves.
If your friend is the one being codependent with you, here is the perfect situation to practice your “I feel ...” statements. Sit down with your codependent friend and say something along the lines of, “When we hang out, I feel as though I’m the one making you feel happy.” If a direct approach isn’t your bag, I would go with then sharing with your friend how similar you both are. The reason why the codependent person acts the way they do is because they are feeling as if the non-codependent person is filling a void within themselves with the relationship. Reminding the codependent person of how they are already whole and complete as they are will at least get them to think about the concept and perhaps get to the point of internalizing it.
Healthy relationships are real and very possible. Any relationship complements your life, rather than completes it. Relationships add value but not ALL the value to your life. Relationships are more than a condiment but less than a full meal. It’s just right. No push and pull. While bumps may be experienced along the road, it’s not an exhausting journey.