After the original issue occured, I did do alot of reading regarding the viewer and LL's stance on this viewer. From what I have seen, heard and read, they don't appear to have any issues with the viewer but, because of the fact that it is open source, if you have modified it to do illegal activities, they will have an issue, which is very understandable.
If your friend has not done any modifying on the Emerald viewer and got it from their offical site, I would say that they have more than enough to go to LL and contest the banning and have that lifted. I use it regularly and I do see ALOT of other residents with it as well and if there was an issue, there is alot of us who would be banned. For using the unmodified viewer, there is no logical reason for him to be banned.