According to my expert, she needs to confirm if it is a 401k or a 403b. If it is a 401k, they definitely cannot require it. They can have an auto-enroll, where she is enrolled unless she takes specific steps to prevent it, but they cannot make it mandatory.
If it is a 403b, my expert says she does not believe they can make it mandatory, but she is not positive, because while they are mostly the same regs, there are a few differences and she has never run a 403b.
The US DOL is the regulatory agency for both. That's who she should contact both to confirm whether a 403b can make participation mandatory, and to file her complaint if they say it cannot (or if it turns out to be a 401k, where mandatory is definitely not allowed).

