I strongly claim that making a man believe he spent the last 6 months in space is impossible. For starters, the missing gravity would be quite hard to account for.
Einsteins theory of GR was postulated and verified using a RE-model. Spacetime might be curved, but a mass as little as the earth does not curve it very much, light is barely affected by it. Even the sun bends light to a maximum of only 1,75 arcseconds, and it is way more massive than the earth.
If RE is true, every astronaut has to be a liar.