You should never add anything other than G12 to G12. You could ask your dealership if they will write something down for you stating that the Prestone caused problems. Any of our testimonies would probably not be admissible. Just for future reference, don't do it . It all kind of comes down to what is in the Prestone (yeah, that sounds simple enough).
G12 is free of certain chemicals (it skips my mind right now) but adding other coolants to it can add silicates and things like that (i think it's silicates?) and it'll mess stuff up.
My girlfriend added that junk to the Dex-cool in her car and it would get hotter than it was supposed to..she flushed (yeah, on her own) the system and refilled with Dex-cool, problem solved. So 'add it to anything' is bs.