herceptin and heart function

Hi there my heroes,
I have a question--my mother has been on herceptin for 3 years for liver mets. She has had a miraculous response. NED throughout her body (except for brain mets, which are under control, but which herceptin has no apparent effect on anyway). Her heart function a year and a half ago was 75%. Now it is 50%. Her onc doesn't seem concerned. What does this mean? Does anyone have any info? I'm worried if she has to stop the herceptin.
Any input is greatly appreciated.
Thank you!