William Shakespeare's play "A Midsummer Night's Dream" is not a didactic work, and thus really is not intended to teach lessons. At the conclusion of the play, Puck emphasizes that what we have seen is intended as entertainment, stating that we should consider:
... this weak and idle theme,
No more yielding but a dream.
Thus if you wish to read lessons into the play, it is important to be aware that they are not in the play itself, but something you as a reader are adding to the play.
Helena is essentially a comic character, a rather silly young woman who is willing to betray friends and family due to her infatuation with Demetrius. Although in the play, the magical intervention of the fairies leads to a happy ending, in real life, without magic, Helena's behavior of betraying her friend Hermia's secrets to chase after a man who is not interested in her would not end well. Perhaps the behavioral lesson one should learn from this is that jealousy and infatuation are not good bases for making life decisions.