How is the tragic outcome of the marriage between Romeo and Juliet foreshadowed in Act 2, Scene 6?

Foreshadowing is when one character does or says something which, in retrospect, we realize gave an indication of things that would happen later in the play. The character usually doesn't know that his or her words are prophetic, but, in this scene, Friar Laurence certainly has an ominous feeling that things are not going to end well for the young lovers, Romeo and Juliet, whom he has agreed to marry. Romeo, too, has previously expressed ominous feelings surrounding his love for Juliet and how it might end for him, but, in this scene, he declares that he doesn't care what will happen to him --

Do thou but close our hands with holy words,Then love-devouring death do what he dare;It is enough I may but call her mine.

This is, of course, an example of foreshadowing. Romeo is effectively challenging death to take him, saying that it will not matter to him if he dies, now that he has at least been able to call Juliet his own.
Friar Laurence responds,

These violent delights have violent endsAnd in their triumph die, like fire and powder,Which as they kiss consume

Again, this foreshadows the tragic end which is to befall Romeo and Juliet. It can also be interpreted as describing the lovers' romance as a whole: something moving swiftly, with one partner fire and the other powder, representing the fact that they come from different, warring worlds. When these two elements connect, they will explode.

The tragic outcome of this hasty marriage is foreshadowed by Friar Lawrence's statement to Romeo that

These violent delights have violent endsAnd in their triumph, die, like fire and powderWhich, as they kiss, consume. (2.6.9-11)

In other words, the friar says, intense and volatile joys have intense and volatile endings.  When these joys triumph, such an end is inevitable, and the end will be swift and destructive, like when gunpowder is ignited by fire.  All they must do is touch for the briefest of moments, and then they consume one another.  This statement seems to foreshadow that for Romeo and Juliet, because their love for one another came on them so suddenly and intensely, it will necessarily end as passionately as it began, and not in a good way.  They may "kiss" but that metaphorical kiss will lead to their ultimate and swift destruction.  At this point, Juliet rushes in, and the friar marries them in secret.