"Avril Lavigne is telling friends that her three-year marriage to Sum 41 front man Deryck Whibley is ending because she tied the knot too young. But as In Touch was first to report, it apparently was her close friendship with oil heir Brandon Davis that doomed the relationship. Last December, In Touch reported that Deryck texted Brandon and told him to stay away from his wife after Avril and Brandon dined together, but her rep denied the incident. This time, according to an insider, it was Brandon who gave the ultimatum. In early September, he told Avril to choose between him and her husband. "Brandon really likes Avril, but he's not keen on dating a married woman," a friend explains. "Ever since they first met, they've had a connection. But Avril was married to Deryck, so Brandon kept it casual." Now that she and Deryck, 29, have separated, the insider reveals that Avril and Brandon are planning to go public with their romance, although her rep says they are only friends and that Brandon had nothing to do with their separation. "They have major chemistry. They could end up getting married," says the pal, adding that the pair have been meeting at his mother's Bel Air estate to avoid being photographed together. They also had a rendezvous on a yacht in St. Tropez this summer, and Avril celebrated Brandon's 30th birthday with him at Koi in LA on September 2, giving him an expensive art book as a present. "They are together," says the friend. "He loves Avril."