I don't think she has any legal authority to force Facebook to stop developing Libra. She probably does have the authority to subpoena Facebook executives to testify before her committee.
I also don't think there is a basis for being concerned that Facebook has a lot of data on its users, when discussing Libra. Most of what Facebook has access to is irrelevant to Libra, and banks have access to much more relevant information about their customers, especially their long standing customers, and they have access to information contained in third party databases such as credit reporting agencies.
The concept of a stablecoin is also not new, and stablecoins have had mainstream acceptance in the crypto space for years now.
There are a lot of reasons to be concerned about Facebook, but IMO libra is not one of them.
The difference between banks having private data and Facebook are the way they use it. Banks use it for your identity, prevent things like money laundry, and other activities that they are required to do by law (KYC). Facebook is a social media company that secretly collects data on its users and uses that data in shady ways.