First, I must thank you for completely destroying my appetite.
Second, can you send me the picture? My email is located in my profile. Thanks in advance.
Third, report them to the health department. This needs to be investigated.
Now... on to your issue. I recommend that you speak with a personal injury attorney about this matter. I'm also going to modify your tag to reflect the personal injury portion of this issue.
The basic elements of establishing a personal injury (or a tort in our lingo) is that you must that the establishment had a duty to your boyfriend (which they did), they breached it (also pretty easy to see), that any injury that occurs to your boyfriend (hopefully there is none) was caused by the restaurant (which it was) and that an injury of some kind occurred (which again, hopefully there is none).
The good thing is that you reported the incident to the restaurant. If you report the incident to the health department they should come out and check the restaurant for any violations. If your boyfriend begins to feel ill, take him to the hospital immediately. The CDC maintains a list diseases which may occur if exposure to diseased rodents occur.(http://www.cdc.gov/rodents/diseases/direct.html)
Tell your boyfriend to keep track of any medical issues (especially bills) that may arise as a result of this incident. If an illness does occur, he would have grounds to go after the restaurant to compensate him for the bills (and probably the psychological damage from getting the heebie-jeebies, don't quote me on that medical terminology.)
I also wouldn't eat there again.
Best of luck. I hope no illnesses befall your boyfriend.
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You'll want to speak with an attorney that is willing to due some investigation into the matter as one defense to the matter would be, "how do we know you didn't put the rat in there and take a picture?"
In my experience, the contaminated food claim is far more difficult to prove than one would initially believe.
This comment is made as general information and is in no way to be considered legal advice. Attorney's making such comments have no information beyond the information provided in the question presented. Additional information is necessary before an attorney can give specific advice. Please seek legal counsel.