Thank you so much to everyone for all the great advice! Sounds like exposing for the shadows and letting the brightness of the arc fall where it may is the best method.
@richard ide, Yes, it is a TIG welder and I had not thought of using an ND filter to be honest. I will give that a try, sounds like a great idea. I did not mention it in my initial post, but the camera will be mounted on a tripod, so long exposures are not an issue.
@dmb, Thank you for the explanation; I will try shooting both with and without the UV filter and see what happens. I'll be using a 135mm or possibly even a 200mm lens quite a distance away from the welder, so I'm thinking protection from flying sparks etc. won't be too big of an issue. Or am I wrong? The type of composition you describe is exactly what my friend is after.
@E von Hoeg and noacronym: I will be wearing a welding helmet and since I will be composing/focusing ahead of time, I won't be looking through the lens either. In fact, I'll probably be looking down at the camera as I'll be needing to bracket/advance/shoot/repeat. Is this sufficient eye protection, or should I consider taking greater precaution? I am quite paranoid about my eyes!
Thanks again for the help!