IMO Auron doesn't tell Tidus because he wants Tidus and Yuna to fall in love - so that Tidus is more inspired to save her. Being told from the start that Yuna is destined to die could quench Tidus's ardor. Auron is acting on orders that he doesn't understand, after all, so getting Tidus emotionally involved in the world is to their benefit.

Also - Keyakku is one of Rikku's brothers. [And Tidus can go thru his pockets - search the body - and find potions; just like waaay back on the ship when Keyakku gave Tidus potions then, before Tidus went swimming to search for the sunken airship.]

[apologies if this was double posted - my account would not show that my reply posted successfully, so I posted it here.]