Steel would be best for welding, probably most of these deco. elements are not steel,..I'm thinking of cast pot metal aluminum type...if they can not be welded then they could mounted by drilling and riveting.

I was checking out King Metals for scrolls: http://steel.kingmetals.com/search?w=scrolls&view=list

They say all their scrolls are weldable. What I don't know is whether any weldable scroll can be welded to any grate, or whether the particular type of metal is important. My friend is not a pro, just someone who does this as a hobby, and he does not know either.

I do a lot of fabrication on homes, not so much with scrolls but I know they come in two types. There's mild steel and aluminum castings, the aluminum ones often have steel tabs cast into them to make it easy to weld to regular steel handrails and columns etc. I would recommend finding an all steel one though as they're going to be the easiest for your friend to work with.