Yes
Can you attack using reach from bo staff and stand still when a medium creature comes towards you. If you have a 10ft reach thx to bo staff it fulfills the trigger however stand still says "you strike out when your foe tries to flee..."
Meeting the trigger requirements is the important part. If they are 10 ft from you (and hence in reach of your no staff), and they then move one square toward you then you get your strike from Stand Still.
Since word 'flee' does mean run away rather than run toward, I understand your confusion. However, that sentence doesn't say 'flee from you.' In this case it makes more sense to read 'flee' as 'flee from their current square.' After all, the name of the feat is Stand Still, not Don't Run Away. ;)