Question about multiplayer leveling of npc henchman

Hi, i was wondering how does the leveling system works for multiplayer campaigns/modules, does any PC in the party can manually level up the npc henchman or only the lead player(the host of the game) can do it?

Let’s put the next example for easier understanding of what i’m asking for:

Player 1 is hosting the OC for Player 2, making the Player 1 the “Lead” Player. They are travelling alongside Kelgar and Neeshka. They kill some enemies and they are ready for level up.

Does only Player 1 can level up Kelgar and Neeshka or the Player 2 can also do it?