@briancornpop
Didn't he resign from his job when they wanted to have him transferred? If that was the reason for his exhaustion, shouldn't he feeling better now?
@briancornpop
If you look at chapter 8, I think, it tells us that he faints everytime she kisses him. That suggests that his poor health is mainly due to her.
Work may also be a factor (as he gets little to no breaks and comes home late) but he was working there before ever meeting her. Thus she's probably is the main cause.