Maybe it would be a better idea to leverage the bad dreams...which are, among other things, ways that we cope with the real world or things in it that we're "afraid" of. I would think that simply denying/blocking someone's ability to have the bad dreams could be worse. We need to learn how to work with how things work, rather than assume they're wrong and try to change them.
Sometimes we're able to process the bad dreams and move forward, sometimes that's not as easy as it sounds...especially for those with PTSD. Forcing someone to have good dreams sounds like giving them a kind-of Advil...it blocks the pain receptors (as I understand it) rather than fixing the actual problem.
Advil is OK for some things, but it won't fix a broken leg.
I now refer you to Murphy.
I don't want to come off as a chicken-little, just want to voice an additional note of caution...no matter how unlikely it might be.
I guess there's lots of things they could do...but all of them have risks...does the reward out-weigh the risk (and the effort)?