If you could upload their game logs (~/Library/Logs/Boundless/log.txt) it would confirm, but assuming it is the same intel-mac crash issue, then it has absolutely nothing to do with going fullscreen.
That you are able to create characters and it crashes when it then goes fullscreen does give some hints as to what shaders are “not” the problem (since everything rendered up until that point in time doesnt cause a crash) though, which is helpful!