Well a high fat diet whatever the origin will invariably lead to weight gain, any perceived benefit from a metabolism boost would always be negated by that. But if you are following a low fat healthy diet, then that's entirely different. You said yourself, your friends eat a lot of oil. Any diet can be made unhealthy by eating high levels of fat as a part of it, vegan, vegetarian, or carnivorous. But healthy spicy food could provide a temporary metabolic boost. It raises your internal body temperature ( as evidenced by the fact that we often sweat more and feel hotter when eating a hot curry ) which means we are working harder to regulate that.