ur topic is confusing too..no offence..he is not lying to arman..if u mean he should tell arman abt the marriage thingy..he did try to tell him in last days epi before araman cut him off mid sentence, sid was gonna say that u ought to know sth abt ridhima bt arman said he knows everything abt her..for ur ref :)
n by not telling ridhima he is trying to help arman, he is giving ridhima full independence to see who she chooses..the guy just wants to know who she loves..n if ridhima found that sid was helping arman in some way, she would just be more confused realizing how much she "owes" sid again..there u go :)