Texas is, in fact, a community property state. You're absolutely right that a married person can give their 1/2 of a community property asset to another person. They could even do so under the terms of a Will, which you don't mention whether or not one is present here. You're also correct that the remaining 1/2 of the community property asset belongs to your mother. If she did not agree to the gift of her property, she may very well have a claim for fraud on the community against the decedent's estate.
The process to assert this claim will begin and end inside of a probate court. You should be aware that aside from the stocks, there may be several other rights held by your mother that are worth preserving and protecting. Your mom needs to sit down with a probate attorney immediately to begin outlining the facts.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.
I don't think that Texas is a community property state. Unless there were unusual circumstances leading up to source for the money to buy the stocks, or a marital contract in place, I don't think there is anything your mom can do.
This is off-the-cuff advice and does not establish a client-lawyer privilege. Nothing I say here can be used to suggest the avoidance of taxes/penalties/interest due. I am only licensed to practice in the state of Florida.