For one... it was in 1999, that's almost 10 years ago! Give the boy a break, Ridzy!
Secondly and most importantly Riddhima must not forget that she already knew Armaan was a flirt before. Remember, she knew about his "diary" and even set him up with some of his ex-gf's... So honestly, it's not like Armaan's lied to her about anything... She should take all this with the right spirit. Armaan loves her and she should TRUST him...or if she has any doubts, speak to him.
So far Dill Mill Gayye has proved to be a good soap with no headaches/K-drama/Ekta-style annoying hassles, here people speak about the issues normally... So I hope Riddhima doesn't get all filmy/heroiney and attack on Armaan because of Rahul's story (btw, does Rahul not have anything better to do...jab Armaan hota hai to usse annoy karta rehta hai, jab nahin hota to bhi uske baare mein hi baat karta hai, seems for Rahul the world begins and ends with Armaan and Muskaan only)!