Depends on your judgement on what kind of person he/she is.
If he /she is not an evil person, go to talk to A frankly, tell A that you regret that you two did not go alone as well as you wished but you cannot take time back. now and you have to have his/her letter, and you are in a situation of needing his hands which will impack your whole future, ask him face to face if he is kind enough to help you by giving you a good, not a bad, ref letter, be careful to observe him/her and judge from his reaction before deciding if you can trust what he promise or , may be reject-which you got nothing to loose. Just my two cents. hope its helpful to you. best of luck!
