El Greco slipped his own portait into one of his paintings.
Your friend should evaluate whether or not the source is primary or secondary. Once they figure that out they should figure out the author and if there is any bias present in each source. Hope this helped
You should most definitely consider the bias of this person's claim. He or she makes a living off of natural gas drilling, and is therefore likely to hide certain facts for their own personal gain.
I believe you are describing density currents
