My friend wants to know which guitar is better in terms of tone.
In terms of spec, the RG421 will be higher, but they have different neck profiles, so tell him to try them both out at a store if he can. If that's not possible, it's probably safest just to go for the 421 since it's slightly higher end.
^ yeah the 421 is the higher end guitar- unless he has a good reason to go for the other, I'd go with the 421.
Also, one is fixed bridge while the other is probably using a cheap tremolo. The 421 will probably sound a little better, but if it has the stock pickups it's best to swap in better ones anyway.