You must not lament in conversation with your savior over the trifles of an average boy.
what does it mean?
To lament means; mourn aloud, wail, express sorrow, grieve for; bewail, bemoan, regret
Trifle in this context means; matter or object of small value or of little importance
In the sentence
your pen-pal's admonishment might be interpreted in two ways.
Finally, it is worth pointing out that in both cases it has been your pen pal who has classified these problems as being trivial and insignificant. Whether she or he is correct in making these rather arrogant claims is a separate issue but one you might do well to reflect on.
Without knowing any of the context about who the "savior" is, or what the trifles are, or who the boy is [you|someone you know|your pen pal], or whether he is mutually understood as average or not,
this sentence seems to mean that:
you should not complain or express sorrow to [your savior|allah|god|jesus|superman] about this boy's problems.
Possibly because the boy is average & thus unimportant, or because the boy's problem's are no different than any other person's problems, or maybe your pen pal thinks you have more important things to talk about to [your savior|krishna|budha|jesus|aquaman]. Or maybe that you shouldn't let this boy's problems become your problems.
In any case, it sounds like bad advice to me, as gods aren't real, so you'd be wasting time better spent taking real action. And even if they were, we should still have some concern for our fellow man.