@Amanda916
I always try to keep the guest informed about various things that could affect them including info about weather, major events which could affect traffic, public transportation strikes or recent taxi/subway fare increases. You could keep it casual and say something along the lines of "Not sure how much this could affect you but I just wanted to let you know there are some fuel shortages happening in Portugal so you should keep this in mind and be prepared while travelling. Have a safe trip~" Although it's not your responsibility imo keeping guests informed would be the right thing to do.