What does Helena do when she learns that Hermia plans to elope with Lysander in A Midsummer Night's Dream?

Helena tells Demetrius that Lysander and Hermia are eloping.

There is a complicated situation going on in Athens.  Helena is in love with Demetrius, but he does not return her affections. Hermia’s father has chosen Demetrius to marry his daughter. Hermia, however, is in love with Lysander.

Helena sees Hermia as competition for Demetrius’s affection, even though she loves Lysander instead.  Helena is jealous of Hermia. 

Call you me fair? that fair again unsay.
Demetrius loves your fair: O happy fair!
Your eyes are lode-stars; and your tongue's sweet air
More tuneable than lark to shepherd's ear,
When wheat is green, when hawthorn buds appear.
Sickness is catching … (Act 1, Scene 1) 

From Helena’s perspective, Hermia has two men that are in love with her and she has none.  She thinks that Hermia has stolen her man.  Hermia tries to assure her that she is not interested in Demetrius by telling her that she and Lysander are eloping.  

The moral of the story is: Do not tell Helena anything.  Helena decides to tell Demetrius what Lysander and Hermia are doing. 

For ere Demetrius look'd on Hermia's eyne,
He hail'd down oaths that he was only mine;
And when this hail some heat from Hermia felt,
So he dissolved, and showers of oaths did melt.
I will go tell him of fair Hermia's flight (Act 1, Scene 1)

Helena will tell Demetrius Hermia and Lysander’s plans, and since the last thing Demetrius knew he was supposed to marry Hermia, he will go after them.  Of course this is how all four lovers end up in the forest together, where Puck causes mayhem by mixing them up, anointing the wrong lovers with his special potion to make them fall in love with the wrong person.  Eventually, Helena ends up with Demetrius and Lysander with Hermia.