Informers, dogs, grasses, fizz gigs... call them what you will, they are the number one reason people who handle drugs get done.
Some do it for cash, some do it to stay in business, some do it for revenge, some do it to burn bridges or escape, and beleive it or not, some do it simply for teh lulz - just to toss a cat amongst the pigeons.
Is it ok? That depends. Everyone's scaling of morality is different. However, just because one uses drugs doesn't mean one should necessarily be more prone to calling police because they beleive they've received bunk gear - there are myriad explanations for your shit being sub par, only some of which come down to your dealer being at fault, and even fewer with your dealer intentionally trying to fuck you.
One common mentality with dogs is this - its not that something went wrong AND they got fucked over, its that they got fucked over BECAUSE something went wrong, and that it was all aimed at them.