Take a look at the sentence again:
Davina, a good friend from my biology class, has more interest in what is
under a guy's skin than what is under his clothes, for
this reason, she is pursuing a degree in
radiography.
The comma between reason and she is perfectly legal. For this reason is a prepositional phrase that is introducing the main clause that follows: she is pursuing a degree in radiography. When a prepositional phrase acts like an introductory transition, as it does in the sentence above, use a comma after it.
Go back to the sentence to try again.