She needs truly bulletproof wheels. I'm surprised by new ruts and potholes continually. I use Performance kevlar-belted tires, cheap at $10, and have only had one flat in the last nine months or so... thesecheap heavy tires seem harder to puncture than a lot of pricier ones... but I often wish I had sturdier wheels.

Despite the previous poster's advice, I'd DEFINITELY put at least a front brake on even if you go fixie - cars are too unpredictable not to be able to stop superfast when necessary, even at speed. That said, I'm not much of a fixie rider, and I know that real experts can stop more quickly than I can.

Use vinyl or electrical tape to black out brands / logos wherever possible.

If you take Msngr's excellent advice about locking the saddle down with an old piece of bike chain, I'd cover the chain in an old bit of innertube, too. This will not only protect the seat stays, but you can also seal the tube closed with vulcanizing fluid from a patch kit - now whoever wants to steal the seat needs a knife as well as a chainbreaker.

When locking up, make sure the chain / U-lock goes around the tire inside the rear triangle and then around a stationary object. The front wheel can be protected with an additional loop of chain or a Krypto cable or whatever. A LOT of riders here lock just the frame or just one of the wheels, and come back to find that they only OWN a frame or one wheel, based on the number of random parts I see chained up on my daily commute!

And have a look at transalt.org - an excellent source on info on riders' rights and responsibilities in the city!

And wish her good luck! Riding in NYC can be a real blast, though the learning curve is sometimes a little steep and brutal if you're not used to traffic

-chris