For Mario, edge-guarding is somewhat difficult, especially against Ridley, a fighter with a really good recovery. Normally, Ridley with go right under the stage, then go straight up to ledge. A back-air as a potential stage-spike is usually a good idea, and is a safe idea. However, there is some situations where Mario can nair, causing Ridley to go under the map, which will cock-block him. Ridley is good at going low, but he can only go in one direction. Use this to your advantage, and try to get him in a situation where he is right under the stage. Ridley's up-b has a bit of start up lag, so options are possible. if Ridley's trying to recover horizontally, nair is a really good option. Ridley's recovery go the highest straight up. Any other direction doesn't have as much distance. So nair-ing him further off-stage after he used one or two of his jumps is a good option. Just, of course, don't get to predictable. Ridley could side-b when recovering horizontally, and not go into free-fall, therefore it a pretty good option for him. Ridley's side-b is a command grab, and if used off-stage, it send Mario and Ridley down, low under the map. This is good for Ridley, as going straight up is where his recovery shines. Mario, on the other hand, not so much. Be careful when nair-ing Ridley away, once you see he is starting to figure out your nairs, jump over his side-b and f-air him instead.
Mario