First, if you know that teens aren’t treated well by other librarians, that’s an issue and is not acceptable. You need to speak up tactfully, to start. If it continues, you need to escalate to get training for those who might mistreating teens (and perhaps for all librarians, while you’re at it).
Second, let’s start by giving the Reference Desk librarian the benefit of the doubt. If it is a question that can be answered specifically within the teen space, the librarian on the reference desk should bring the teen in and introduce them to teen librarians. After all, they are there to help teens, and perhaps that teen just needed to be pointed in the right direction – they will be welcomed and assisted there.
If it happens again, lead by example, show positive patron service skills (in front of the other librarian), and perhaps suggest they try to help the next teen that comes along, with the promise to help them if they need it.
If it becomes a problem, politely suggest they handle the teens like any other patron, as they’re going to be adult patrons soon. If it still continues, speak to higher-ups to discuss protocol and the need to respect each other’s time off from the reference desk. If those managers are unwilling to help, that might be a clear message to you about how they feel about teen patrons as well.