It's generally better for people to install GrapheneOS themselves since it's very easy and saves a lot of money. It takes 10 minutes to install GrapheneOS with the web installer. It also avoids needing to trust a company to do it, although it's possible to verify an install done by someone else is genuine with the verified boot key fingerprint and/or Auditor. An existing install by someone else should be factory reset it to avoid any sketchy configuration.

Getting a new phone is very useful to someone that's a victim of a controlling partner but it doesn't particularly need to be a GrapheneOS device. GrapheneOS has features useful for this including Auditor and standard Android profiles but we're not specifically recommending it for this ourselves. People who are victims of this probably just need a new phone of any kind and to prioritize other things.