cabriolet

I have a friend that has a VW Cabriolet. When she was driving it she jerked it by shifting in the wrong gear do to a weard trans. Ever sense then it barely runs! I mean BARELY! Someone said they thought it was her fuel injection systems might have gotten cloged form junk in the gas tank. Anythere person told me it sounds like she bent a valve or shaft or something not good. What does it sound like it could be?
Right now as I type this we have the battery charging (haven't messed with the car for about 4 months) to see if we can try a fuel injection cleaner. Will the injection cleaner help? Is it worth the time?
Any other ideas or advice would be greate!
Thanks!
(also posted same question in the general section, didn't see the technical section, sorry moderators)