It ain`t all true.I do 50 min. cardio every day and i don`t sweat so much and i go fat fast and i don`t gain wait doin it like that.Your heart rate should at 24 to 25 beats per 10 seconds to burn fat.You don`t have to go crazy jogging like Jim Fixe.Do a moderate speed with little incline on the thread mill and you`ll feel the diff.
I would recommend low-intensity for 30-45min a day.
(walking up an incline on a treadmil, slow pace, 3.5)
Can't specify a heart rate for you, but your friend is wrong. You don't have to sweat nor do you have to huff-and-puff to get maximum fat-burning. In Ketosis, any cardio will burn fat.
To be in fatburning range you should be at least at 60% of your MHR for at least 20 minutes. You can calculate your MHR the old way 220-age, or the new way 208-(o.7xage) (I think)