If the joint account holder used the card and the primary account holder paid the bill for a period of time that is the strongest evidence in the defense of this type of case. Normally, if someone tries to prosecute another who has had a relationship with them the state looks very carefully at this. In economic crime cases, intent is the key to prosecution. For example, if this person was accused of taking from other accounts belonging to this putative victim, that can be used against them. But if these two people had combined their bills for a period of time that is also a defense...remeber, criminal theft is different than civil conversion which this looks to be. Please consult a criminal defense attorney in your state as each case is different. Most criminal attorneys will see you free of charge...take advantage of this.
Fraud is actionable if certain elements of proof are established by the evidence. Using the written documentation you have is a good place to begin. Contact a local lawyer - many may give you a free consultation for an hour - to discuss your specifics. Far too many variables exist in the short post you wrote for any further observation by me.
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