"Metallic" paper is nothing new and has been around for a while. The paper isn't actually made of metal, and I don't know why the paper is actually described as "metallic." Maybe someone else can chime in. Many of the better photo print shops offer this type of paper, and is usually only slightly more expensive than say a "luster" or "gloss" type of paper. I have ordered prints on "metallic" paper for Adorama, and the do look kind of "metallie" for lack of a better term. I gues thats why they call it metallic.
That said, there is a process for printing images directly onto a sheet of metal. I believe aluminum is typically the type of media it is printed on. Adorama just recently began to offer this type of print to its customers. (Maybe within the last two months) They print the photo directly to a piece of aluminum. Their pricing for this type of print is VERY expensive.
My thought is that this is the type of print your friend viewed at the gallery. Assuming that it is, the gallery owner was probably just "puffing" that only one place does these type of prints, to give greater value to the images he is trying to sell, or he may not actually no of any other places that actually offer this service.
I believe this technique of printing on aluminum is a fairly new process that most people just aren't familiar with yet. But I have seen several shops online that do offer it.