I'm not modding D:OS 2, but it should look like this:

Code
IF
ObjectFlagSet("EventName", _NPC, _ID)
AND
DialogGetInvolvedPlayer(_ID, 1, _Player) 
THEN
CharacterSetFollowCharacter(_NPC, _Player);
ObjectClearFlag(_NPC, "EventName");


For setting character flags in dialog see this wiki-entry:

Dialog editor (the relevant information should be under: Node Actions).

My mods for DOS 1 EE: FasterAnimations - QuietDay - Samaritan