Another thing is digitizing. I do not know how "techie" your future M.I.L. is, but if she does use the computer and the internet, she can go through her things and digitize them. Say, Angelene mentioned receipts from stores - you can scan them or you can capture them on a digital camera or your phone. Maybe if you create a digital collection for her, she would be able to part with those items.

Clearly, you cannot digitize bulky physical objects, but receipts, pictures, newspaper clippings would all be good candidates for digitizing.

Maybe your son can explain to his mom that it is much easier to find a scan of a receipt in her digital collection than to find a receipt in a pile of junk.