Yes, this can work. However, like all other forms of brain or mind alteration, it has some irreducible risks. One of those is that it's always going to hit some unintended targets, because memory is more holographic than discrete. The other is that it's going to take a fast five minutes for any mindwipe drug to hit the streets,and then you have two very bad problems: 1) people blowing their brains out with it while trying to self-medicate, and 2) people using it to erase someone else's memories. 0_o
This doesn't mean don't create an extremely precious treatment for PTSD. Just know what you're getting into.