Proper punctuation can do wonders for a sentence. In my opinion, this sentence sounds jerky and needs better punctuation.
“She was there, with him; that was all that mattered.”
Since with him is important to the meaning of the sentence, the prepositional phrase should not be separated by a comma. In addition, the sentence sounds smoother when the clauses are joined by the coordinating conjunction and.
She was there with him, and that was all that mattered.
