This is why my and the g/f always go out together!
However, had anyone done this to her then god help the stuff id do to them. Having a really bad temper and jealousy problems does come in handy sometimes! I once broke a lads nose just because he slapped her backside, i was a lot younger then and had nearly no control over my emotions! Im older and have much better control.
However, in your situation i would probably go to the builders yard with her and get her to point out the builder, take his picture and then go down to the police station and make a report of sexual assault against him.
As for the tramp, theres not a lot you can unless you go back to the same spot at the same time to see if he's there.