What do you want to know by this "what can cause this" ? Since when he has heart failure? This can be a side effect of chemotherapy, can be due to viral infections, or some other cardiac cause. It seems he has been managed nicely as per standard. Presently as per his primary treating hematologist or oncologist he is not fit for further chemo, should be given supportive care.
Next Steps
follow the advise given by cardiologist and hemato oncologist