How do we convert a legal separation into a divorce in Ohio?

My fiancé and I have been together for almost 2 years and he has been legally separated from his ex for 2 1/2 years. They have no children together and have no health or financial benefits with each other. We have no way of getting in contact with her because she is not the most stable person. She is in and out of jail frequently for minor misdemenors and does not have any place she stays at long enough for us to get her served with divorce papers. We would like to know how to turn his legal separation into a divorce or if there is another way for him to get his divorce without her because we don't know where she is. They do have a signed dissolution by both her and him but we were told that in order for the courts to serve the dissolution they need a physical address for the papers to be served at and she needs to show up to the appointed court date. Please if anyone can help us get his divorce so we can get on with our lives we would be forever grateful.