Well there isn't a direct bus to Dijkzigt, that's for sure and as far as I can see none of the other routes come close enough for someone out of town to figure out where to go. I'd say take a cab right at the Ahoy. You could also take the subway at Zuidplein, right across the Ahoy to Beurs and catch a cab there, but money-wise it won't make much of a difference.

[Edited 7/7/11 8:00am]

Subway line "Erasmusline" will run all night because North Sea Jazz. You can take that line from "Zuidplein" (opposite Ahoy) to "Beurs". If the other line "Calandline" is not running, you have to walk 2 stops to "Dijkzicht", which is opposite Stayokay.

Good luck.

No not correct rherscheid. The subway will not stop at Beurs after 01.00. You should get out at Leuvehaven and then walk to Dijkzigt. Its not far. After 01:00 the metro will only stop at:

Rotterdam Centraal – Stadhuis – Leuvehaven – Wilhelminaplein – Maashaven – Zuidplein - Slinge.
So thats the blue line on the picture and only those stops that i mentioned above.

Well in that case he might as well get a cab at the Ahoy.