I think the above writeup suffers from a misinterpretation of Sartre's point. It is not completely wrong, rather it stops short of the final logical connection needed to understand the existentialist dilemma. At this point, I might offer the disclaimer that I have not read the story in question; I trust Glowing Fish to have accurately presented it.
Sartre is confirming the core of his presentation of existentialism, often conveniently expressed as "Man is condemned to be free." The important point of the story isn't Sartre's explanation of the risks and benefits of the two choices for his student. It's in Sartre's inability to offer advice.
As an existentialist, Sartre saw that it would be wrong to make the decision for his student. Talking about the good and the bad doesn't undermine this view one bit. The good and bad matter only tangentially, compared to the student's need to accept the responsibility for his decision. Responsibility requires knowledge, and that brings us full circle, explaining why Sartre laid out the risks and benefits of the options. It also explains why Sartre didn't discuss the other options: the student had already eliminated them, so there was no reason to.
Sartre's big revelation seems to be that there is no magic book, written either in ether or gold leaf, that can tell us what to do and after which we wouldn't have to feel any worry, doubt or regret.
Yep. That is Sartre's big reveal. Sorry you're underwhelmed.
Also, I feel that the Kantian analysis is flawed, in that the maxim wouldn't pass muster in the categorical imperative. It contains far too many subjective terms (meaningful, immediate), and ultimately drops out of the system because everybody has obligations all of the time.