Yes, it is completely possible to port most landline numbers to a cellular carrier. Reversing the process and porting back to a landline carrier would be a bigger challenge.
The port-in process is the same as for a wireless-to-wireless port. They will need to provide the name, address, and account number, along with any authentication information (account password or PIN, etc) from the original service provider, and should NOT discontinue their landline service first. The landline service will disconnect on its own after the port completes.
Landline-to-wireless ports usually take considerably longer than wireless-to-wireless ports, which are usually almost instant, and almost always under a few hours. Landline-to-wireless ports take anywhere from a few days to a couple of weeks (generally around five days).
It's normal for the landline to continue to give a dialtone and make outgoing calls for a couple days after the number is active on the wireless phone. It will eventually disconnect.
Make sure they don't have anything that relies on landline service before porting out (satellite or digital cable boxes that need to be connected to a phone line, a home security system, or DSL service).