Originally posted by @Varinder Kumar:
How do I go about persuading him not to do it.
Put some pants on and tell him "No".
Yes, welders are a huge fire and safety hazard. They make things into red hot MOLTEN STEEL! I say this as a pretty accomplished welder myself. I've always had a "workshop", but no, that's not something I would ever expect to be allowed to do in a rental unit garage. Yes, they do cause fires and injuries all the time, just because you haven't personally experienced that means nothing.
Sparks go flying when you weld, especially if you aren't good at it. Do you know how good your tenant's welding skills are? Is he going to chase down every spark and make sure it's out? Or is he going to get thirsty, and wonder in the house leaving a piece of red hot steel behind?
Even if there is no fire there is a very, very serious chance of very serious burns. Yes, things get hot in the kitchen, but that is not even remotely the same thing. It will also create a lingering burning metal smell in the garage...do you allow cigarette smoking in the garage? It's not much different.