Until they are separated and have divorce action pending their assets are joint, (that is they are both of theirs). Who paid for the tickets is immaterial.
If husband had used wife's ticket for his own use (for a mistress for example), wife might be able to claw back the value of the ticket as part of the property settlement portion of the divorce.
It sounds like your mom needs to get a lawyer though as it appears there is much she needs to learn about divorce law. These situations are very volitile and often need the cool head of a professional. They also depend heavily on facts and the ability to capture and proove facts within the Rules of Evidence.
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