Depends on what tests she had done and what the results were - but yes it could be heart failure; without knowing the specifics there is no way to tell. Diabetes can be very detrimental on the body. Ask her doctor for explanations and make sure she understands it all; dr's sometimes won't take the time to explain it all.
If her heartbeats are 15 and should be 60, she may benefit from a pacemaker which will regulate the rate of her heart to a preset rate and hopefully help her - also if it is heart failure she may need meds to help with that.
Here's the forum on MH that deals with heart failure a bit more:
http://www.medhelp.org/forums/Heart-Disease/show/72