Since it is a limited edition print (5/6) and obviously old, I'm guessing it could be a woodcut or other hand-produced print. The significant acid burn on the border indicates previous framing with materials inferior to today's standards. So, if this print is valuable to its owner or considered to be collectible, then a trip to a conservator would be advised, to work on the acid burn, as well as to flatten it.
Considering the degraded condition of the paper, attempting to flattening it by heat or moisture infusion might cause more harm than good.
If this is not particularly valuable or potentially collectible and the owner is not willing to pay for restoration work, then you could make it flat using an acrylic DCO mount under a new mat, with a secondary layer of glazing on top.