Hello and welcome to JustAnswer. Please note: This is general information for educational purposes only and is not legal advice. No specific course of action is proposed herein, and no attorney-client relationship or privilege is formed by speaking to an expert on this site. By continuing, you confirm that you understand and agree to these terms.
I am sorry to hear about this situation. On this website, I do not always get to give good news, and this is one of these times.
You are a volunteer. As such, someone in your situation is then not covered under ADA (Americans With Disabilities Act). Under Title I of ADA, only individuals with disabilities who meet the definition of "employee" are entitled to reasonable accommodations.
Even though the other person is being allowed to go to the scene, that does not mean that the department has to allow you to do the same, because as a volunteer you cannot demand participation in events, and are not covered by the ADA so argument about disability accommodations is moot.
Please note: If I tell you simply what you wish to hear, this would be unfair to you. I need to be honest with you and sometimes this means providing information that is not optimal. Negative ratings are reserved for experts who are rude or for erroneous information. Please rate me on the quality of my information; do not punish me for my honesty.
I hope this helps and clarifies. Please use the SEND or REPLY button to keep chatting, or please RATE when finished. You may always ask follow ups at no charge after rating. Kindly rate my answer as one of TOP THREE FACES/STARS and then SUBMIT, as this is how experts get credit for our time. Rating my answer the bottom two faces/stars (or failing to submit the rating) does not give me credit and reflects poorly on me, even if my answer is correct. I work very hard to formulate an informative and honest answer for you; please reciprocate my good faith with a positive rating.