In "会很  的"， is 很 meaningless? 会~的 explains the future event that will happen.

她肯定会很高兴的。
She will be (very) happy.


Is that 的 a collaboration of 会~的?(很 does NOT mean 'very', like 我很好)or just associated with the adjective 高兴？ (很 does mean 'very')

Thank you so much in advance.
I don't know exactly what do you mean by "meaningless", but the sentence would definitely sound weird without “很”. (I wouldn't really use the word "meaningless", because basically every character in Chinese comes with a certain "meaning", no?)

“的” is necessary in this constellation with “会”. 

她肯定很高兴。
She is for sure very happy.


or

她肯定会很高兴的。
She would for sure be very happy.


I'd say the first sentence is less common compared to the latter.