Yes, HIV can be transmitted by invasive procedures such as colonoscopy if the instrument is contaminated with the virus from another procedure. However, every good hospital sterilizes its instruments before use. HIV is a rather fragile organism and is easily destroyed by many disinfectants. Though theoretically it is possible to transmit the infection by a dental instrument, it is extremely unlikely if no procedures are done.
The CD4 count in the HIV infected person can be quite normal. It starts to fall when the body's immune system is severely compromised and the stage of AIDS is near.