Gene meets Leper's mother when he goes to Vermont to see Leper at home after receiving his telegram about having "escaped" from the army. She appears for the first time immediately after Gene has kicked Leper's chair, throwing him to the floor:

Quick heels coming down the stairs, and his mother, large, soft, and gentle-looking, quivered at the entrance. "What on earth happened? Elwin!

Realizing that Gene is responsible for her son's being knocked out of his chair, Mrs. Lepellier confronts him angrily: "Did you come here to abuse him?" Gene apologizes, while he and Leper's mother help Leper from the floor. The tenseness of the situation eases when Gene starts to leave, but Leper asks him to stay for lunch.

Mrs. Lepellier's attitude toward Gene warms during lunch, and it becomes apparent that she has forgiven him for his earlier behavior:

Mrs. Lepellier began to be reconciled to me because I liked her cooking. Toward the end of the meal she became able to speak to me directly, in her high but gentle and modulated voice . . . when she offered me a second dessert, I saw she had accepted [my apology]. "He's a good boy underneath," she must have thought, "a terrible temper, no self-control, but he's sorry, and he is a good boy underneath."

Her changed attitude toward Gene is demonstrated after lunch when she suggests that Gene and Leper take a walk together.

In "A Separate Peace" how does Gene feel about Leper?

His feelings change quite a bit throughout the course of the novel.  At the beginning, he is mildly amused by Leper and all of his nerdish ways; he tolerates Leper, is kind to him to a certain degree, and even defends him against bullying on occasion.  He feels sorry for Leper because he is such a nerd, and pities him.

Later, after he visits Leper after Leper is released from training camp, he first feels pity and sorrow at Leper's predicament.  But, that is soon replaced by fear and repugnance.  He fears the way Leper is behaving, is terrified of the accusations Leper is making, and even hates him a little bit as he leaves.  It is a hatred born of his own hatred for himself; Leper reminds him of what he did to Finny, and that makes him angry.  Gene doesn't want to think about it, and dislikes Leper for reminding him.  He also resents Leper's clinginess and desperate need for his friendship; it is pretty intense, frightening, and pathetic in his eyes.  He also fears Leper because he is an ugly and unwelcome picture of what war can do to a person.

At the end of the novel, his hatred crystallizes as Leper reveals the true events of the tree incident; however, Gene is so wrapped up in Finny's next accident that there is no time to dwell on it.  He moves on.  Leper is just a side-tale in the main tragedy of Finny's demise.

I hope those thoughts help to get you started; good luck!