'90 Oldsmobile Eighty-Eight Won't start. Old gas? Oil?

My boyfriend's grandparents bought a '90 Olds 88 from their friend, while he saved up
the money to buy it from them.  It sat in their garage for a year or two, and he recently
made the exchange with them and has been driving it for a week or so now.  It's been
starting kind of  rough, so he would have to depress the gas pedal a bit to get it catch.  
I told him I was worried about what the bad gas could do to the engine, but he didn't
really want to go through the hassle of siphoning it, and he doesn't know a lot about
cars, so I told him he should keep topping it off with new gas and hopefully it would
eventually get rid of the problem.  The other day we were driving it just fine and turned
it off for a bit, but when he went to start it, it just wouldn't go.  The engine turns over
and when you depress the gas pedal it starts for a few seconds but then dies
immediately, so I know it's not the battery or alternator.  I took a look under the hood,
and when the car ran for a short period of time, the engine block would shake and
sound pretty rough.  We are still in college and basically broke, so avoiding a
mechanic is ideal.  Any suggestions would be greatly appreciated!  Thanks!