This bridge crosses the river, next to the Church of the Angel Raphael. The bridge is never named in the book, but it is significant. It is where Julia first meets Nicco, where she suddenly realises the truth about Carlo, and where she makes her final crossing.
On her website, the author identifies this bridge as the 'Ponte de Cristo',(Bridge of Christ) a name which is perhaps suggestive of divine guidance. Click on the link, then click 'view map' and hold the cursor over no. 16 to read the details.
A picture of this unimportant-looking brick bridge can be seen here
The old man could be a Jungian archetype, representing perhaps wisdom, or even God.