It is not illegal to build a service with the same functionality.

So you are going to have to prove their code is a copy of yours, not just a copy of the functionality.

In Google vs Oracle, APIs also aren’t necessarily copyright able:

“So long as the specific code used to implement a method is different, anyone is free under the Copyright Act to write his or her own code to carry out exactly the same function or specification of any methods used in the Java API. It does not matter that the declaration or method header lines are identical”

To sum it up; get a lawyer.