It's a great question, and a tough one to address. You are absolutely correct: this person is definitely still married in the eyes of God and is therefore not available to date, let alone free to marry in the Church. Regardless of there being a good chance that this person will receive the annulment, there should be no dating. This person cannot assume the annulment will go through. And even if it does go through, there is no guarantee that the tribunal will permit that person to marry. It does happen sometimes that a person granted a decree of nullity is NOT permitted to marry. Typically, this is a temporary situation due to the Church determining that the person needs additional counseling for a period of time before being permitted to marry in the Church.
While this is going on, I don't see anything wrong with being a friend to someone, or developing a friendship. After all, friendship is a cornerstone to any relationship, including marriage. But obviously, friendship does not mean that it becomes more than a friendship. My advice is that if you enter into friendship with this person, do NOT allow your heart to become invested in any romantic way, or your mind to have any expectation that this person will be free soon to become more than friends with. So that means that your friendship cannot take on any kind of "dating" approach. Get to know each other. Offer prayers and support for what he is going through. Share with him things in your life. Pray together. If you get together in person, make it with some friends, or maybe meet for coffee. Nothing romantic. Just very friendly. NEVER touch each other, if you can help it. I know friends hug, but this friendship has hidden behind it a waiting for something in circumstance to change so that it might become more. Therefore, it is not right to allow it to develop TOO deeply even at the friendship level. After all, if you get involved with a man who is free to marry and you are interested in, it won't be right that you have this friendship with this other man. Even though nothing more than friendship had happened, it still is not going to sit well with any man you date that you have a close relationship with another man. That would be uncomfortable.
On a final note, you should consider dating other men while you are developing this friendship with him. That will help ensure for you that you are not "waiting around". If you don't date for other reasons, so be it. It's just a suggestion to help you keep your motives pure and keep things "real".