There are many spoilers and its mis-leading to believe which one could be true. I don't think that Neil would have cheated on Ragini. I do believe that Roopali is Neil's ex girlfriend and Suhani must be Roopali's daughter with someone else.Possibility #1 : After all the years, Roopali must have returned and claimed that Neil is the father for Suhani which Ragini may have believed and caused the separation. This could have been a plan by either Roopali alone or planned with Pam to separate Neil and Ragini. So this could be the scenario if Roopali was bad.
Possibility #2 : Perhaps Roopali was good after all and Roopali returned into Neil's life and maybe she was just a friend but was mis-understood by Ragini. Ragini's mind could have been poisoned and was made to believe that Suhaani was Neil and Roopali's daughter leading Ragini to suspect them. Maybe the happenings at that time eventually leaded Ragini into believing its true. Not being able to bear the blame of causing a rift between Neil and Ragini or embarrassment, Roopali kills herself and Neil feeling guilty for it decided to raise Suhaani.
Lets see what story do the CVs play out.