220BPM - AGE = MAX HR
MAX HR x (% desired) = Target Heart Rate.
Many schools of thought on target HR for fat loss. I recommend trying various rates to see how she responds.
I suggest if she is just starting out to target at about 65-70% Max HR for starters for 20 to 30 mins. Maybe introduce intervals as well.
My philosophy is that if you start out with something that does not overwhelm her she will be successful and therefor be motivated to continue.
Of course after this post 20 guys will tell you HIIT is the only way.