Can Oracle SE2 RAC works with only a single instance? Is it recommended? I've got a client with an Oracle DB 12 SE2 RAC with two instances.

One of the instances is giving too much problems. He wants just to remove it, and therefore leaving Oracle RAC running in only with one node.

Is the client forced to convert its DB to RAC ONE NODE or to Single Instance?

Regards!
You can just simply shutdown and disable one of the instances.

RAC One Node is optional in this case. If you want to use features like online database instance migration or rolling patching, you may use it, this is a simple configuration change, there is nothing to be converted.

Keep in mind, while RAC and RAC One Node are available without extra cost with SE2 in 12c, starting with 19c, these options are not available anymore in Standard Edition (SE2).