If they take leave and travel to or transit through a long list of countries that their employer has said "do not travel to", and they contract the virus, they have been advised that they will not be paid sick leave. So will either have to use holiday leave or LWOP.
That sounds very dubious. I can't even imagine how that would fly (boom tish) if they specifically had this as a condition in your terms of employment / contract let alone an EBA. It also isn't upto the employer to make decisions on paying sick leave based on a medical opinion. If your doctor says you're sick he simply notes you're sick, he isn't required to tell your employer the condition that has caused your sickness!
The employer doesn't seem to be making rational decisions in this context.