So we have "Luego él se la mostró a sus amigos" and I learnt that "se" replaces "les" to avoid alitteration. But do I have to include it at all? I have already specified "a sus amigos".
Maybe you should ask that in the Spanish forum :)
(For what it's worth, I have no idea what the "se" is doing there, but I never got far in Spanish ;-) )