Demetrius's previous love affair with Helena complicates his desire to wed Hermia first by casting doubt on his sincerity. (It is important to note that when Lysander says that Demetrius "made love ... to Helena" those words don't carry our contemporary meaning of a sexual relationship. They mean he wooed her, verbally declaring his love for her.) Demetrius's prior love interest can only raise audience sympathy for the plight of Hermia and Lysander: not only is Hermia not in love with Demetrius, he comes across as wanting to wed her on a whim. More importantly, Demetrius has wooed Helena successfully: she is in love with Demetrius and determined to have him. This complicates the plot by adding a person to the mix beyond Hermia and Lysander with a desire to stop Demetrius from marrying Hermia.
Hermia reveals to Helena her plan to elope with Lysander because she and Helena are good friends, and she doesn't want her friend to despair. With Hermia out of the way, Helena should have every chance to regain Demetrius's affections. Hermia obviously doesn't expect Helena to reveal the planned elopement to Demetrius.
Demetrius, I'll avouch it to his head,
Made love to Nedar's daughter, Helena,
And won her soul; and she, sweet lady, dotes,
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man.
I must confess that I have heard so much,
And with Demetrius thought to have spoke thereof;
But, being over-full of self-affairs,
My mind did lose it.
It's not so much the fact that Demetrius has slept with Helena that is the problem, but the fact that it is being openly talked about by everyone - even Theseus has heard about it. It makes Demetrius seem rather less than gentlemanly, and it makes everyone think differently of him - marrying Hermia off to him would be much easier if he was thought to be whiter than white.
The Elizabethans also had a kind of belief in virginity before marriage, though, in fact, this seems to have been more stringently applied to women than to men.
2) Why do they tell Helena? Well, notably, it isn't Lysander who tells her, but Hermia. And why does Hermia do it? Because Helena is so upset about Demetrius' being in love with Hermia and not with her - she decides to try and comfort her:
Take comfort: he no more shall see my face;
Lysander and myself will fly this place.
Before the time I did Lysander see,
Seem'd Athens as a paradise to me.
Hope it helps!