Undetectable means that no HIV virus can be detected in your blood. For this to happen you need to take all of your HIV medication as prescribed by your doctor and not miss any doses.
The short answer is no. All of the research done now says that if you are undetectable, then you cannot transmit the virus. Studies have shown us that when one partner is HIV positive, undetectable, and taking medication as prescribed and the other partner is HIV negative, the virus does not get transmitted through sex. This is the biggest study that proved it
They will need blood tests regularly with their doctor to verify that they continue to be undetectable. They need to test undetectable for at least 6 months to say for sure that they are undetectable.
PrEP increases prevention of HIV . Check out PrEP Page for more information.
You are still at risk for other STIs. Still use condoms, use lube and get tested.