Just tell her: sweetheart, I would love to help you, but...

"But I think maybe there is some miscommunication between you and the developer and since communcication is essential for teamwork and added-value, I would strongly encourage you to schedule a meeting with xxx (the original developer). I trust that xxx is competent for the job assigned to her and same for you. If there are any major different opinions between you two on specific issues, maybe you can schedule a meeting with her along with the manager so those issues can be discussed in a bigger picture. If you would like me to be present for that meeting, I would try to fit it into my schedule. In the mean time, good luck and I'm sure this thing will work out one way or another. So don't sweat about it and have a great day!"

Basically, a lot BS and there is not much she can say about it. Save the email and even if later she says anything to the manager, just forward the letter to the manager and there isn't much the manager can complain about the letter.