The 320s aren't that old. I just scanned the first 10 full pages of thread titles of the P320 forum on Sig Talk and did not see one that was about rust so it must not be a widespread problem. I don't doubt your friend's has a problem, and that others have had similar, but I am guessing it will get fixed or replaced if sent back.
Or you know, you could just google "sig rust issues" and see all the results, instead of hoping for objectivity on a brand specific forum. I wouldn't look for objectivity about HK's on HKpro or objectivity on glock from glocktalk
Page 6, only one i saw about finish issues in the first 10 pages of the 250/320 section.
https://sigtalk.com/p250-p320-p320-x-5-pistols/354546-bad-qc-new-coyote-m17-s.html?amp=1
Not all sig owners are required to post about their issues on sig talk, so just because you didn't see anything on that board, isn't concrete proof that it isn't an issue.
People have been joking about sigs finish issues for close to 10 years, amd after seeing a $600+ and a $900+ dollar gun develop rust, one to the point of pitting...
Yeah, sorry, its going to take more than 10 pages of sig talk to change my mind