The coming of the NBN doesn't stop Foxtel Cable from working. Foxtel may decide to end Foxtel Cable in a particular area and migrate customers to satellite. I know people who have been asked to migrate some time ago and others who haven't been asked even now. So your neighbour shouldn't worry about this until it happens, if it happens.
If your neighbour can upgrade to the NBN on the same package and at the same cost then they should do that.
Otherwise they should keep Foxtel cable, upgrade to the NBN and they can cancel Foxtel or switch to Foxtel Now or something else later if they want.
You should check whether the Foxtel package is bundled in with the NBN package. If so they are switching away from Telstra then you may want to unbundle them, but check any minimum contract period requirements.