My father isn't a citizen, he needs to travel every six months, I wanna know, can he just go to Puerto Rico instead of mainland, and I've heard about the ferry, how much does it cost round trip?
Going to the US or US a US territory such as Puerto Rico periodically for short times will not necessarily allow him to keep his residency unless he is fortunate.
Residency means that he must actually live in the US or he could forfeit his residency. I know that many Dominicans with US residency visit the US periodically within 6 month periods and many do get away with it, but that is actually in violation of the regulations.
The regulation(law) reads that if you are out of the country 6 months or longer then it is the option of U.S. immigration whether or not to allow you back in again.
Some people are not aware of that information.
As Windeguy stated, U.S. immigration one day may give the traveler an option on the next visit, stay in the States or lose the residency. This has happened to friends of ours.