Why is there a "had been" in this sentence - English grammar question.?

[Firstname]

"I realized that social media would be transformational for businesses and decided to start a software company that would help Fortune 500 companies manage their marketing efforts using social media. Steve Garrity, who 【had been】 a computer lab partner of mine at Stanford, left his job at Microsoft to start that company, Hearsay Social, with me in 2009."
If I substitute "was" for "had been" then what's the meaning of the sentence and what's the difference between the sentence with "had been" and with "was"? Thanks in advance!
SirGeorg..., do you mean if I substitute "had been" for "was", it will be wrong because people who read the sentence will think that when he was my partner and working at Microsoft, he left school and the job to start a company with me?