Gary Price is a member here on BHO. I used to be a dealer for Price cases, they are bulletproof. With their overlapping rubber valance closure they are pretty close to air tight. Corrosion is a result of humidity. And since the shell is fiberglass rather than wood, there is nothing to regulate trapped humidity in the case. Does your friend live in a high-humidity environment? If so, don't store the instrument in an airtight case! And/or, consider adding a dehumidification system (some sort of desiccant) to the case but this needs to be monitored so as to not dry out the air too much.

Here's an article I wrote on the topic about 20 years ago:

https://web.archive.org/web/20070128102044/http://www.finecases.com/about/humidity-control.html

I have several Price cases, and have not had any negative issues with them. I also have several other good high end cases, Calton, Hoffee, Cross Rock, Leaf, etc. - have not had any issues with any of them that would seem to be related to premature corrosion issues on the banjos or banjo parts directly related to the cases. I have personally seen (and reading about or hearing about) some premature tarnishing of banjo hardware on the 70's era Gibson banjos, and seem to remember they had built (or more likely, had built by contractors) cases where they may have not given sufficient time for the glues used in putting those cases together to cure properly, and it may have been responsible, OR actually had some cases that had materials or glues used in the manufacturing process that simply did NOT get along with the plating on the metal parts or the finish, and caused premature tarnishing of metal parts or some other issues with the finish on the wood parts.

I had one Calton case that worked well when my main banjo was in it, and I was opening the case almost every day.

When I switched to a different banjo in a different case, and put the former one away for several months there was corrosion on the tuning pegs, seeming to come from underneath the gold plating.  And there was heavy tarnish on the capo and the tension wrench in the case pocket.  I changed to new tuning pegs, nickel, those corroded too after a few months in a closed case.

My guess it was something in the glue, maybe in the fabric, building up in the air in the case.  As long as I was opening the case regularly, no harmful build up.  But when the case stayed close for a long period of time, then the tarnish/corrosion began.

Other two Calton cases, and one Mark Lear case, no problem, whether banjo was used regularly or stored away.  Same environment, same humidity (which is regulated well in our house.)