The spark from a spark plug is very hot at the grounding point, but also very minuscule. By itself, it would have no chance of affecting a valve or valve seat.

A burnt valve is typically caused by excessive carbon buildup, incorrect ignition timing, incorrect valve lash, or poor AFRs.

I haven't pulled a ton of BMW heads, but the ones I have always surprise me with how much carbon buildup they have around the bases of the stems. I would venture a guess that some of them were due to the crappy valve seals BMW chose.

If your mechanic is correct in it being a burnt valve, you would need that head rebuilt. yes, "technically" you could fix just that valve/seat but the cost of just doing the whole head vs. the time/cost to R&R the head would make it prudent to have everything gone through. But just as importantly, I would want the cause to be tracked down.

Was it carbon buildup? A poorly flowing injector? Incorrect valve clearance? Etc..

I would also ask them how they diagnosed it being a burnt valve/seat.