Lepellier does suffer mental trauma in the army and does escape before his discharge. Below are three quotes that support that assertion.
When Gene questions Lepellier on how long his furlough will last and when he has to return to the army, Leper finally admits he is absent without permission: he has escaped without a discharge:
“I didn’t get any pass,” he groaned; with the sliding despair of his face and his clenched hands, that’s what it was; a groan.
The above quote shows how anguished Leper is over the whole experience.
“I escaped!” the word surging out in a voice and intensity that was not Leper’s. His face was furious, but his eyes denied the fury; instead they saw it before them. They were filled with terror.
The second quote again confirms that Leper acted on his own initiative without waiting for the official discharge paperwork. It also reiterates his trauma: he is angry on the surface about the experience he had in the army, but beneath that he has been terrified. He was unprepared for the reality of the army.
In the quote below, Leper explains why he left the army without leave to do so by explaining that he wanted to avoid a Section Eight discharge:
A Section Eight discharge is for the nuts in the service, the psychos, the Funny Farm candidates. Now do you know what I’m talking about? They give you a Section Eight discharge, like a dishonorable discharge only worse. You can’t get a job after that.
Leper shows his trauma by being fiercer and more aggressive than he ever was at Devon, as well as being more frightened. But his words show he thought through leaving and did so for logical reasons. He did not want to be dogged for the rest of his life with a kind of discharge that would make it difficult for him to get a job after the war. He uses terms, such as "psycho," that were not common at the time and make Gene feel uncomfortable.