So your friends just want to have a little extra income?

It's ok to start with.

But if I were to design the system from scratch, I would plan the way to take over all the booking system in China within 6-12 months, including Sina and eLong.

I don't think he can move that fast with limited resources. The bad site in China is that there is no netwoeked travel booking system like Sabra (am I spell right?) in USA, but the positive site is that your friend can grab this opportunity to create the model (ok, copy one) from the existing system.

Where does he want to stand?