How about a copy being made? Giclee?
Fire retardant isn't what is the problem as much as the extreem heat that flows out and up, washing the FRONT of the oil.
If it is something that they enjoy and it won't be passed down.... I wouldn't worry either.
My aunt bought a Robert Wood in the early 60s, and it has been over the mantle ever since. It's in fine shape as is the frame.... but in Southern California they only light a lite fire about 6 times a year if that. But, also RW is sort of passe these days, and when she goes, it will probably go to her son and over a mantle again.... they bought it to enjoy in their lifetime.
Sometimes in our rush to protect and concerve, we forget that our customers are RARELY museums.
I could care less what happens to my Hiroshigi prints when I croak. And because I framed them about 35 years ago, they probably are backed with cardboard.... but then I'm 52 and as my eyes dim I won't be seeing the Foxing anyway.