make a mould of the other side
use some clamps to make it supportable
fit it to the bent side
use a heat gun to heat up the plastic, but not too close or else it'll melt
fit the mould over the top
tighten it up slowly but not too tight
let it cools down natuarlly
see how much it's moved
repeat until fixed

i did this to a cav gsi bumper that had hit a lampost and bent in half when i fitted it to my 3 door and it came up a treat