People often gain weight from chemo. The steroids make you eat like a beast.
Stomach cancer in someone who is 21 is nearly unheard of - it's a rare cancer in the first place, and only 1% or so of the time does it happen in someone in that age bracket (and frankly, in that age bracket, the older would be more common)
http://seer.cancer.gov/statfacts/html/stomach.html
Usually radiation is daily only for a matter of weeks, not months.
Smells like BS to me - her folks don't know? C'mon....