I'd want lower temps for long term use personally, for me high 70's is the cut off point on acceptability.
If he has the stock fans in the stock configuration in an s340 that means he has no front intake fans at all (the s340 for some,reason comes with 2 fans both fitted as exhausts)
If this is the case then tell him to get 2 x 120mm (or better 2x 140mm) fans fitted in the front of the case.
With a 250w gpu like the 1080ti you need constant airflow.
By the sound of it he has none.
Also there is no reason to manually ramp fans up & down in afterburner, set the custom fan curve up properly & he should never have to touch it again.
actually that wont help .. the closed front panel must e removed for this to work .. i toled him to try to take the top exhaust fan and put it in the bottom front of the case .. hope this will work
thanks for you help =D