Although shingles are contagious and can transmit the chicken pox virus to susceptible people, classic, localized shingles are not as contagious as chicken pox itself. While like shingles, you can get chicken pox "by touching or breathing in the virus particles that come from chickenpox blisters," you can also likely get chicken pox "through tiny droplets from infected people that get into the air after they breathe or talk." The droplet spread doesn't happen with shingles, though. You typically have to have direct contact with the shingles blisters for it to be contagious. The American Academy of Pediatrics even states that 'lesions that are covered appear to pose little risk to susceptible individuals.'
Still waiting to hear what our doctor says (it's hard to get through to him) it sounds like it's safe for the boy to see his great-grandmother but she should avoid rubbing his spotty bits.