Ciara was major in 04' and 05' but during the "Evolution" after Promise she started to fade coincidentally around the same time Rih got ha first #1 with SOS.

Did the good sis Rih contribute to He's downfall?

No. Her fading was largely due to the platform she began with; she was established with a super trendy sound (the Atlanta "crunk" era) and when that sound fell out of favor, it kind of left her without a lane so to speak and that was made even worse when R&B in general began to fall out of favor with the mainstream as well.

No. Ciara started flopping because the niche that was carved for her flopped too (crunk). Rihanna had nothing to do with it, as she was always more diverse & they didn't make the same type of music until like 6 years ago.