She could either:
A) Hide this fact and use this opportunity to make Pragya the villain in Abhi's eyes (again)
B) Go straight to Aaliya, who will then (probably) find Bulbul and attack her or find a way to attack her family emotionally
C) Go tell Abhi
I feel like Tannu is a very selfish character. A little less than Aaliya (who is actually crazy) but selfish. I don't see her going up to Abhi and telling him the truth. Since Abhi has already started warming up to Pragya, this could end up badly for her.
So in the end I could see her going up to Aaliya or hiding the fact from everyone and scheming something on her own.
To be honest, I would like to see Tannu snap at Aaliya because it was her crazy idea, she didn't even get her facts right. Also... Aaliya never seems to think about anybody but herself. She snapped at Tannu today who couldn't even say a word to her.
But either way, this misunderstanding isn't going away any time soon. I just hope Purab isn't dragged back to the mandap.