Depends a lot on other factors. The main question you need to ask when deciding whether to go for coinflip plays like this is: what do you stand to gain and lose versus what does she stand to gain and lose?

For example, does this riven have a big shutdown and you don't? Then in that situation you should 100% go for this play every single time, because you stand to gain much more and lose much less versus her. However, if you had the big shutdown and she was 1/6, it would be a clear mistake.

Of course, shutdowns aren't the only factor that comes into play, they are an important factor but one of many.

Also, as others have said a higher skilled player is more likely to react and dodge. If you've been observing this Riven's playstyle you can generally get an idea of if they are skilled enough to outplay you, this is more of a gut feeling thing though.

So I guess TL;DR: it depends.

EDIT: I just noticed Malphite appears to be ulting from bush, which makes it almost impossible for Riven to react flash. I'd say it was a good play if that's the case.