I guess it's too obvious to mention, but the tuners ("professional" ones, standing by their reputation, in particular) should (must, according to the local consumer rights law?) provide some warranty on their work.
I must admit it's extremely strange to me tuning not only invalidates manufacturer's warranty (on the parts tuned only, they say), which is obvious, but allows an official service to refuse any (paid, of course!) help at all.
Like undoing a tune and returning the car into a "manufacturer valid", working state, for example.