Best months: well whenever between Oct to March or April.
We got wing last year until May but the best / safest months are probable Nov to Feb.
Sand sharks are no pb Dude: They are friendly and cannot harm.
Wetsuits / water temp: The water is freezing' in Cape Town (kite beach / sunset / big bay) therefore you will need a 4/3.
Langebaan is quite a bite warmer but I'll still recommend a 4/3 through.
If your girl friend is uncomfortable in waves, avoid kiting at kite beach where the swell can be very rough (with loads of kites on the water).
Big Bay has a section (close to the rocks) which is more sheltered from the swell therefore it could be worth a look.
Langebaan: No pb to kite there, no hazard (watch out for the out-coming tide in SE winds at main beach through).
All the best,