Oh, this discussion has happened many times. There are lots of posts about this in the archives.
Thing is...just yesterday someone (forget her name?) posted that the first time they were going to do the liver flush, a relative came over and surprised her. So she had fasted and taken the Epsom Salt drinks...but not one drop of oil passed her lips because her relative showed up. The next morning..she passed some stones! (I've heard of other accidental flushes, like when Tiratu on her 29 day Master Cleanse fast, started drinking carrot juice to come off it and passed a whole bunch of stones).
So, in my books....that's pretty bottom line: Nope the oil and lemon or grapefruit doesn't combine or cause the stones to form...