truck won't start

OK guys my grandmother is trying to sell my cousins truck for him and she calls me today and says it won't start. I go and try to jump it off but that doesn't work. The truck is getting juice but absolutely nothing to the starter, it won't do anything. The interior lights,dash cluster lights and headlights flash intermittantly. I'm kinda thinkin it's the securilock antitheft system, but why, and how do I fix it if that's what it is? She has one of the original keys to the truck, is there anyway she could have done something to the key to make it unrecognized by the anti-theft system? If you guys could help me out before my cousin kills my grandma I'd appreciate it. BTW it's a 02 lariat

disconect the battery for awile than try it