In the two years since Ms Yang joined UBS, it has been the joint bookrunner on two large bond sales and a key adviser on the two largest public acquisitions ever involving China Resources, which employs 400,000 people and boasts assets of more than $120 billion.
Mr Song, 51, who sometimes goes by the English name "Charley", was detained late last week as Chinese Communist party anti-corruption officials announced he was being investigated for "suspected serious violations of Communist party discipline and the law".
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His detention came days after a journalist working for China's official Xinhua news agency publicly alleged that Mr Song had used his mistress, Ms Yang, as a "channel" to receive huge bribes and launder money.
Writing on his Weibo microblog, the journalist, Wang Wenzhi, also alleged that Mr Song "used his influence" to get Ms Yang hired at the Swiss banks.
Before he was detained, Mr Song denied the allegations in a statement posted on China Resources' website that has since been removed, along with almost all other references to him or his nearly three decades at the company. All traces of Mr Song have also been removed from the website of Hong Kong's Independent Commission Against Corruption, where he was chairman of the Hong Kong Ethics Development Advisory Committee.
A statement from Mr Song that has now been removed from the website said in part: "I firmly believe that ethical business practices and good corporate governance are the cornerstones for maintaining a level playing field and also sharpening our competitive advantage."
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Ms Yang could not be reached for comment and current and former employees of UBS said they were not sure of her whereabouts.
China Resources, Credit Suisse and UBS all declined to comment, but one person from UBS who declined to be named confirmed that Ms Yang worked there and said the bank was looking into the allegations and the circumstances under which she was hired.
A person working at the bank who declined to be named said that UBS had a prior relationship with China Resources before Ms Yang joined, as evidenced by several non-fee-paying, non-deal roadshows it conducted for the company prior to her appointment.
Ms Yang was hired by Credit Suisse in 2009. Before then, the only business it had done with China Resources was to advise on restructuring of China Resources Gas in 2008 and to act as bookrunner and lead manager on a follow-on equity sale by China Resources Land in 2007.
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But in 2009 alone, Credit Suisse acted for China Resources subsidies as bookrunner and lead manager on four follow-on equity sales and one IPO, according to Dealogic data. The bank also worked on a China Resources follow-on equity sale in 2010.
While Ms Yang was at the bank and apparently in charge of its relationship with China Resources, Credit Suisse also won the mandates to work on an acquisition and a bond sale by subsidiaries of the state conglomerate.
Before Ms Yang joined UBS, the total value of the three China Resources-related deals that the bank worked on came to less than $850m.
But the value of the two bond sales and two acquisition deals UBS worked on for China Resources after she joined adds up to more than $7.7 billion.
Since banks usually earn fees based on the size of the transactions they handle, this suggests UBS's income from China Resources-related deals increased greatly after Ms Yang joined the bank.