Your LFS is an idiot. Like all rays, the reticulated ray does have venom in its babe sheath.
If untreated, it cause necrosis of the skin and bacterial infections, up to and including amputation of the limb or death. These are extreme cases.
Like previously posted, submerge the affected area in the hottest water you can handle without burning yourself. (Scalding burns generally will be more severe than the venom at this point).
The venom is a protein based toxin, so hot water helps break down the protein within it.
Still go to the doctor to have it checked out. There is a medical sticky in this subprime with exact info on what to do, as well as medications used to combat it.
An xray of the sting area is handy too as it confirms whether or not you have any fragments of the barb still inside, as these can lead to infections also.
Your LFS needs a damned good slap on the back of the head for their terrible advice.
