Should she keep the promise?

So about a year ago my friend made a promise to a boy who dated her best friend before he died. She promised him no matter what she would protect her. After his death her best friend betrayed her though, she went with her other friends and left her behind. This tore her apart but no matter what she said she wants to keep the promise. Her former best friend gets sick very easily to the point she has to go to the hospital frequently. When they still talked by friend always went to the hospital to visit her constantly. After his death I noticed she would always watch her and her other friends talk at another table, shed watch her walk away in hallways to. She always checks her Facebook to, to make sure she hasn't put any status' about her not feeling well or anything. When i'm on a walk with her and her dog she always passes by her house and stands there watching it for a few minutes before taking my arm and keeps walking with me. I noticed around her house her dog has been acting strangely as well, just like her, her dog would watch the house until she decides to go walk again. One time she saw this strange man grab her former best friends arm out in town and she suddenly just went up to him and kicked him to the ground and threatened if he ever touched her again she would kill him. So I took the chance to call the cops and they took the man away. I saw her former best friends shocked look but my friend didn't look at her, walked back to me, took my hand and walked away. After the promise, she hasn't been herself. It's like shes tearing herself apart in order to keep her promise. I'm really worried about her, should I try and convince her not to keep the promise she made?