What my Krishna friend discovered is not that the Gita was wrong but that it should not be an excuse for getting away with anything. His guru, Bhaktivedanta Swami, had a fundamentalist and selfish way of interpreting the Gita. Duty under him therefore was entirely a restricted to HIS way of thought duty, aka Dharma.
I personally see an incredible sophistication and beauty in the Gita philosophy, which when compared to an equally sophisticated view of Christian philosophy reveals many parallels [not identifications] but one has to be careful. There is a deterministic quality in the Gita that can, like I said, cause one to think that one can get away with anything as long as one thinks it is "my truth", "my path" "my dharma." That immature selfishness is not the case in essential Hinduism. Just as Christianity can lapse into off doctrines like predestination under Calvin, so can Hindu sects fall from the grace of the philosophy as with Bhaktivendanta.
Stupid is as stupid does.....to quote an American icon's mom.