If the Wedding is October 30th, I guess you'll be in The Netherlands in the end of September the start of October? that can be a good period, with a bit of luck you'll have nice weather... if you're unlucky it will rain non-stop
overall I find Rotterdam a horendous city but it does has its nice places and the harbour is very cool. but since your friend lives there he'll know what to check out.
The city centre of Amsterdam is also worth visiting, don't here me complaining there
however, I do suggest you visit other parts of
The Netherlands (Holland* is only the western part of it and consists of two provinces

)
If you stay within Rotterdam and Amsterdam you're idea of The Netherlands will be rather limited.
I suggest you either pay a visit to the north (Leeuwarden/Groningen) the south (Breda/Eindhoven) (most southern city is Maastricht but it's very annoying to get there) or, my personal favorite, the east (Enschede).
I suggest you'll travel there by train, that's the easiest way, I find, to get a good impression about the landscape
*It's a bit of a touchy subject for Dutchmen who don't live in either North or South Holland if a foreigner refers to The Netherlands as Holland. we might have a bit of a Inferiority complex about that