I have a friend in the USAR. He is an officer with no obligations. Can his unit stop him from going into the IRR? What is the IRR regulation?

Where do we find the IRR rules? I have an Officer in the USAR who wants to leave his unit. He put in his IRR packet over a month ago. He did all the counseling already and he wants to get out. The Officer can't take his promotion because there are no slots and he wants to go to the IRR to get his higher rank and to serve abroad (Title 10)in a slot that requires the higher rank. He has TS and speaks 3 languages. He is getting no real progress. He could go to the IG but he rather go to the IRR and manage his career elsewhere. In today's army, retention sometimes screws over excellent soldiers who have the ability to deploy. The Higher Command, in my opinion, worries more about their numbers yet they won't promote their best due to office politics. I am not in this situation but my friend is. I am as happy as a clam!