Those words "archivally mount" disqualify the majority of attachment options. First, the attachment should be non-invasive and completely reversible, which eliminates all adhesive mounts, except maybe torn Japanese paper hinges held by starch paste/methyl cellulose/Klucel G. But then, the old, fragile, deteriorated paper probably could not support its own weight from a couple of small contact points, and would tear itself from the mounts on minor impact or any kind of stress. Dry mounting and wet mounting are very permanent, so out of the question.
Also, if you could find an acceptable mounting technique for the paper, it would react badly to any exposure to radiation - such as visible light, or even worse, ultraviolet. That would accelerate the discoloration and embrittlement of the paper.
Plan A: Do as Ylva says - frame a high-quality digital reproduction, make a couple of extra copies for the owner, and suggest that he keep the original in a proper storage container.
Plan B: If you absolutely must frame the original, you could encapsulate it in Mylar-D, which would provide overall support with no adhesive in direct contact, and you could show the edges of the sheet in a mat window cut about 1/8" larger that the sheet. Framers are often "not happy with the appearance", but most consumers accept it when they realize that museums often frame fragile papers that way, and when they understand the benefits.
Plan C: Frame it using a Direct Contact Overlay (DCO) of fine-mesh fabric (Crepeline) or acrylic. Instructions and several cautions are explained in the book.
Plan B or Plan C might be OK for support and reversibility, but make sure your customer understands that the old newspaper will continue to discolor and deteriorate, and at a much faster rate than it did during those decades when it was stored in a dark, temperate place...maybe in a box, well insulated from ambient changes under other papers? (just guessing)