In Germany, can he start a lawsuit against his employer?

One of my German friends has been hired by a company in Köln 4-5 months ago. He is able to perform his job remotely and therefore he had asked his employer if he can move to Poland and work remotely but unfortunately, his employer rejected this. Yesterday, my friend furiously told me that he thinks of starting a lawsuit because some people in his company work from home remotely but he is not allowed.

I am not really familiar with German law but in any case, his employer would not want to work with him anymore. Besides, I really don't think my friend would won the lawsuit unless it is clearly stated in his contract that he can start working remotely anytime. Am I correct?

In Germany, can he start a lawsuit against his employer?
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