The caption is “the night couldn’t have been better with you girls” but I was wondering, shouldn’t it be “without you girls”? Because you couldn’t have had a great time “without” the help of your friends. Am I right?
Your understanding of the words is correct; if something couldn't be better then it was unsurpassably good, or perfect.
Your parsing is off, however. It isn't
[the night couldn't have been better] [with you girls]
[the night .. with you girls] [couldn't have been better]
Whether the participation of "you girls" was necessary for the goodness of the night was required is incidental to their presence at it.
No, the sentence is correct. Couldn't be better or in this form, couldn't have been better is an idiom.
According to Free Dictionary, its meaning is:
(One's well-being or the state of something) is perfect, exactly as desired, or better than expected.
So we can interpret this sentence as follows:
The night was perfect/exactly as desired/better than expected with you girls.