"so" used as a conjunction

Should I treat "so" in the following sentence as a conjunction and place a comma before it (after the word "window")? I cannot think of a grammatical reason for not putting the comma there, but I feel it would be inappropriate and would unnecessarily disrupt the flow of the sentence. Also, I believe "so he could catch a glimpse of her" is restrictive, making his desire for her to look out the window clear; and for this reason, I do not want to set it off with commas.

He prayed the girl would look out the window so he could catch a glimpse of her, but she never did.