Red giant might be impossible, but a collision with a [white dwarf][1] could kill a star. But I think the most likely result would be a supernova and a neutron star remnant. The mass limit is only 1.4 solar masses after all. I guess you could assume either a low mass white dwarf or a collision with high enough energy to blow off much of the mass of the sun. In any case the addition of the white dwarf would allow the composition of the sun to change relatively fast and it would add heavier elements and probably blow off significant amount of hydrogen, so it would drop the lifespan.

Downside of this solution is that the "special effects" would be spectacular, quite deadly, and entirely different from the red giant you asked for. But if that is okay...


  [1]: http://en.wikipedia.org/wiki/White_dwarf