I assume its pretty simple and can be done with a trip to the DMV.
also, as long as all his paper work is straight....is his registration current on both trailer and boat? does he have the titles? are they in his name? are they clean?
I'm sure you will have to pay for new tags in CA, but it should be too expensive.
I can bet you will spend more money and time just getting the boat home, then it will to get the paper work in your name.
so in other words....go for it!