It doesn't "prove" it, it merely establishes it as a possibility. They could have came up with the idea independently.
Even so, talking about what Melania did isn't the same thing as putting out a photograph.
Actually it is more likely then not since the words and phrases are the exact same. Thats all thats needed. This is not a high burden of proof. need only show its probably true and its more then that.
When combined with Cruz’s other behavior issues this campaign it shows a pattern and practice of sleazy politics and blaming others. After a few times even the staunchest Cruz supporter, if honest is going to question his actions where hes doing the Sgt Schultz impersonation over and over and over.