He really should do something to treat the metal though fire starts an aggressive oxidation that will eat the metal in short order,fire damaged cars rust away shockingly fast if nothing is done to stop it.
When I had my shop, the only way that we could fix fire damaged metal was to cut it out and replace it. No matter what was done, we couldn't ever get paint to stick to it for any length of time.
Fix it, that kind of patina is not "in" in my book. When I caught my Max Wedge on fire I should have just left it in it's burnt state? I don't think so.