The ring tech may not keep someone together, but its a object symbolizing the promise two people are making to each other. It could be anything, your friends boyfriend chose a ring that his girlfriend happens to like. I say good for them. Anything showing commitment from anybody is okay in my book. I respect them for still believing in that kind of stuff.
It is no more or no less significant then any other ring given to another one. Can things change in this relationship that make them break up or grow apart? Yes. Can that still happen if they get engaged or married? YES. Happens all the time.
The ring is a start, and a sweet gesture, if they like then fantastic. The rest takes real commitment, confidence and belief in ones self and another.
To me if your in any relationship there is a commitment to be faithful to one another, thats what the promise ring symbolizes... again its just a symbol that these two are saying they will remain faithful to one another as long as they are in a relationship.