My clients with similar surgeries benefit great from Pilates. It is gentle and provides the core work, as well as flexibility they need.
Initially we work on range of movement without pain with very light resistance. Core work is mostly isometrics, incorporating lots of deep breathing.
Once the client reaches a comfortable level in Pilates I am incorporating light arm work with weights.
I would recommend to look for a Pilates teacher that has experience with breast cancer survivors.
This link might help you find someone in your area. http://www.pinkribbonprogram.com/