successful_befriend_critical_change_effect

successful_befriend_critical_change_effect = {
	$CHARACTER$ = {
		hidden_effect = {
			if = { #Remove potential rival relation if you have it
				limit = {
					has_relation_potential_rival = root
				}
				remove_relation_potential_rival = root
			}
		}
		if = { #Set friend relation
			limit = {
				NOT = { has_relation_friend = root  }
			}
			if = {
				limit = {
					exists = scope:relationship_reason_location
				}
				set_relation_friend = { reason = $REASON$ target = root province = scope:relationship_reason_location }
			}
			else = {
				set_relation_friend = { reason = $REASON$ target = root }
			}
		}
		else_if = { #Fallback
			limit = {
				has_relation_friend = root
			}
			$CHARACTER$ = {
				add_opinion = {
					modifier = friendliness_opinion
					target = root
					opinion = 40
				}
			}
		}
	}
	if = {
		limit = {
			has_government = landless_adventurer_government
			has_perk = friendly_counsel_perk
			$CHARACTER$ = { is_ruler = yes }
		}
		add_contact = $CHARACTER$
	}
}

As written in game/common/scripted_effects/00_scheme_scripted_effects.txt.