Reference letter without permission
As the student’s supervisor, it is your duty not to “help him get the job” but rather to act in a supervisory role, and that includes giving your honest opinion about the student to anyone he requests you to. When I get requests from students I hardly know to write them letters, I tell them honestly there isn’t much I can say other than what class they took with me and what grade they got, and that a letter from me would not be worth very much, but I always say I am willing to write the letter if they insist that I do.

My suggestion is that you simply answer the reference request from the employer as honestly as you can. If there isn’t much you can say, say so, but include whatever details you can speak about that may be of interest to an employer. If you really think you can’t say anything of value, say so. That would probably hurt the student’s chances, but that’s not your problem.

Finally, I wouldn’t mention to the employer the story about the student putting your name down without your approval. It may be annoying, but I would attribute it to the student’s inexperience and just let it go, although perhaps I would privately mention to the student that such behavior is counterproductive and also somewhat rude.