In the poem, how does the speaker's wrath affect his friend and his enemy individually?

The speaker's wrath hardly affects his friend because he "told [his] wrath," and so "[his] wrath did end." Evidently, he talked to his friend about his anger, and, as a result, his anger subsided. However, with his foe, he "told it not," and so "[his] wrath did grow." He continued to harbor his wrath, holding a grudge against his enemy rather than discussing it as he did with his friend. He tended that wrath as one might a prized plant: he watered it with his fears and tears, and he sunned it by being deceitful. It seems, then, that he not only did not speak about his wrath, but he actually hid it and lied to his enemy's face. As a result of all the attention and care he gave his wrath, it grew into something truly harmful and, ultimately, took the life of his enemy. We can see, then, how short-lived anger can be when we simply talk about our feelings and give the other person a chance to make it right, like the speaker does with his friend; conversely, we see how dangerous wrath can be when we hold it in, keeping grudges, and nursing our anger against another, as the speaker does with his enemy.

In Blake's "The Poison Tree," the narrator expresses his wrath to his friend. Because the narrator (the "persona") has verbalized his anger and unburdened himself, the anger goes away.
With his enemy, however, it is a different story. This time, the narrator doesn't express his anger. Instead, he lets it fester, and, as result, it grows. As the poem says, the narrator "waters" his anger with his fears. By pretending that everything is alright and by smiling at his enemy when he really feels furious, the narrator grows his wrath.
The wrath the narrator feels and nurtures produces a poisonous, deceptive fruit that kills his enemy.
The moral of the poem is that it is better for us to let our anger out before it can grow out of proportion and become poisonous.