It's a little late for the BB. If it wasn't in a legal BB configuration before January 1st, it's an illegal AW. Of course, if there's no proof either way, who would know?
Does he have a BB already? They are very hard to come by, now. The PRK Arms one was the best and allowed one to pivot the action sufficiently for field stripping with the BB in place.
As far as swapping selectors goes, you just unscrew and pull out the magazine catch pivot screw and then remove the magazine catch, being careful not to let the spring fly out. You then put the spring in the new catch and put the magazine catch in its place and force the spring into its place (again, being careful not to let the spring fly out). He then has to work the tip of the new magazine catch pivot screw (the one for use with the bullet button) through the slot in the bolt hold-open device. It needs to go under the plunger. Once it goes through there he has to push the magazine catch just into the right place to allow the pivot screw to go through its hole in the magazine catch, after which it will go through the side of the receiver and then through the hole n the bullet button, which needs to be slid over the magazine catch once the screw has gone through it. Typically, a nut will then screw onto the other side, fixing everything into place.
As far as magazines go, any high capacity magazine already legally possessed in CA will remain so until the end of the month. Before July 1st, he needs to either convert them to 10-rounders, surrender them, sell or otherwise take them out of state, or sell them to someone with a hi-cap mag permit to remain compliant. Disassembly and storage of the parts separately might fly, but I don't really know for sure. Installing a magazine block should be sufficient for conversion to 10-rounds, but of course opinions will vary. They have filed for an injunction in the lawsuit challenging the mag ban, asking for it to be put on hold until the case can be heard and decided. We'll see if that goes through.