I have a palm tree that was laid over at a 45 degree angle due to Hurricane Matthew. I jacked it up straight and filled the gap in the soil at the base of the tree Quikrete in order to help hold the tree up. It is about 8 inches deep and only around 1/2 the circumference on the base of the tree. My brother visited and told me that this would kill the tree. Is this true?

NO !😮! CONCRETE !!!
Get it out !!! Support tree with 2 x 4s or something till roots can get established.
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Tree roots need to breath.

Are you in FL? With sand? If so, I think you'll be okay. I've seen plenty of palms growing with trunks smack dab next to concrete sidewalks, concrete patios, driveways etc.... You say the concrete is only about 8" deep & 1/2 way around the tree. The roots should go under the concrete & be fine. If you are in FL as I suspect, the sand will allow the roots to breathe.
However, if this happens again, then the best way to handle it is what Philipwonel says.

Tree roots dont grow through cement. Quickrete is a plastic cement.
Tree only has half its roots on one side ! Right ?
Whats going to happen ?
Next storm season , with rain and high winds ???

😎😎😎

Palms have a really nice "ball" on them which produces many, many, roots sort of like flexible straws. They will grow down & then spread out beneath the concrete.

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http://www.palmtreepassion.com...

Scroll & read through the whole thing:
http://www.marriedtoplants.com...

" Change a mans thought against his will ! He's still of the same thought ! "
Quote: Dr. Myron K. Downing, PhD, MSW, LMFT.
He's a very wise man.
Also author of book : Taking control of your life.
Great book. But ! Be carefull !!!!
You may learn some things !
Its on-line !!!
You must read cover from cover.
😎😎😎