Simon - as you know, humidity can be the cause of real problems for prints and other paper artworks. Museums go out of their way to create suitable environments for their works - generally, I believe this is between 16-19c and around 50% humidity. From what I've read if humidity gets to around 70% that can start causing problems with condensation and ultimately mould developing.

I'm guessing it's not going to be possible for your dad to control the environment at a constant 50% humidity.

Good framing and hanging is obviously the absolutely key thing for prints in unusual climates. It's all the usual things: conservation materials, print framed away from the glass, air circulating around the print, frame properly sealed, hung out of direct sunlight, etc. I'd go and see a Guild Approved framer and chat it through with them if you're particularly concerned.

Ultimately, I can't imagine that the humidity in France is so bad that you can't come up with a solution you guys are comfortable with.

I am sure Beejoir can offer some tips on humidity / heat, etc, as he is in Asia quite a bit.

Y