Event 51 will not improve you as a smash player in any way as far as technique or strategy is concerned. As it is impossible to play that event like you would against a real person.
As far as improving reflexes? That is a very strange thing to suggest. I suppose that you could use it to try and avoid all of their attacks at once? I guess? But honestly nothing will help your reflexes in regards to smash more than just playing against real people. Preferably people as good as you, if not better than you.
So basically, your friend has no idea what he/she is talking about. Just play smash. Like, a lot. That will be what improves your reflexes. Besides, reflexes are most definitely not the be all and end all of smash bros. Mew2King states that he only has average reflexes, and most top players are not known because of their reflexes, but instead their deep knowledge of how the game works and their ability to predict and adapt to their opponent.