Learn English – What does the phrase “woman who does” mean

Score one for dumb guesses. Specifically in the link you've provided, it's a reference to Ghostbusters (as in, the film) :D

Roughly translated, it's along the lines of "that's a really bad thing you're about to start doing/are in the process of doing"

Specifically, early on in the film there's a scene and some dialog which introduces the concept of "crossing the streams" as a Very Bad Thing. The relevant quote is on IMDB here in all its glory and here's the video clip.

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

You must not lament in conversation with your savior over the trifles of an average boy.

your pen-pal's admonishment might be interpreted in two ways.

• 1. The pen pal is saying not to waste your savior's time over petty, insignificant details. The aches and pains of a romantic relationship with an ordinary boy are unimportant and trivial compared to the magnitude of events, concerns and innumerable requests he has to deal with on a day to day basis. The message implied is that if you are to "pray" or beseech his guidance then do so for things which are more important. Your pen pal does not say what these things are or may be.

• 2. The meaning is similar to the one posted by @Ace Frahm, that your grievances over the problems of a boy may be experiencing, an average boy at that, is of little importance to your saviour. The word, "average", in this context I would read very negatively. In other words this person, according to your pen pal, is mediocre and ordinary and his problems are of little value and consequence.

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.