Lysander calls Demetrius inconstant—or unfaithful, to use a more modern word—because he quickly dumped Helena in favor of her friend, Hermia. Lysander's deeply unhappy about this, not just because Demetrius' behavior is so completely unacceptable, but also because he himself is head over heels in love with Hermia and doesn't want any rival for his beloved's affections.
Lysander ran away to his aunt's house with Hermia, where they were to be married. But thanks to the mischievous Puck, their plans for wedded bliss are put on hold after the sprite accidentally sprinkles love potion on Lysander's eyes while he sleeps. He had intended to enchant Demetrius instead. So when Lysander wakes up, he falls in love with Helena and forgets all about his true love, Hermia.
Demetrius eventually gets the love potion treatment himself, and falls for Helena in a big way. The difference, however, is that Demetrius had already dumped Helena before this happened, a clear indication of his inconstancy, whereas Lysander has only rejected Hermia and fallen for Helena because of Puck's love potion. So there's no hypocrisy on Lysander's part when he urges Hermia's father Egeus not to give his blessing to his daughter's betrothal to the two-timing Demetrius.