how did your friend see Lennier as exploiting Delenn's dependence on him?

According to him (paraphrased for better coherency):

Lennier makes all his emotional bonding moves when Delenn's in distress. He creates a relationship which is inherently strongest during crisis situations by vowing to never leave Delenn's side no matter what. This means that Lennier is seen as strongest---refusing to leave, protecting her---when Delenn is at her weakest, and further, is at his closest to her when others are most far away, abandoning her.