DISCLAIMER: I do not own PJO or BtVS.
The first time Nico Di Angelo met Xander Harris, they were trying to stop an apocalypse. Not exactly the best place.
The second time, a bunch of Slayers had dragged him in as an evil Necromancer and Xander took one look at him, remembered his face, though not his name, and told the Slayer squadron to set him free.
The third time, Nico saved Xander from a group of empousae, by summoning some skeletons and ordering them to kill the demons while he patched a badly beaten Xander up.
The fourth time, Xander saved Nico from a group of fyral demons and repayed the favour by doing the same thing. They actually knew each other’s names by now.
The fifth time was when Vi bought Nico home as her boyfriend, so that Xander, as the One Who Sees, could approve him. Both men took ten minutes to stop laughing, and ended up going for a drink.
After that, Vi made Xander switch the schedule on chores in the Slayer house so that she only had the simple ones, because she’d set him up with a non-demonic boyfriend.