I don't think you need to know where the plane started from, given that you know that it's an 8 hour flight.
Idk... But the answer I got was this
If Laura departs from 9 AM and her flight is 8 hours long... then
9AM + 8hrs = 5PM on Friday, at Rome
If it takes her a maximum of one hour to retrieve her bags and go through customs in Rome, it'll be 6 PM in advance of Siena...
If Laura proceeds to go to Siena, then it'll take her approximately 2 hours to take the train. It is more likely to tell Brittany to expect to see her by 8 PM on Friday.