Heffernan, who won the Northern California Classic, was joined atop the leaderboard by Graham Delaet, who carded a five-under 66. Those two are tied at nine-under-par 133. Adam Speirs posted his second straight 67 and is alone in third at minus-eight.
Stephen Gangluff notched a three-under 68 and is alone in fourth at five- under-par 137, while Antonio Maldonado, Mike Grob and Anthony Rodriguez share fifth at minus-four.
Heffernan flew out of the gate Friday with back-to-back birdies at Club de Golf El Tigre. He moved to three-under with a birdie on the fourth, but gave that stroke back as he bogeyed the par-three sixth.
The 30-year-old atoned for that mistake with a birdie at seventh. Heffernan birdied nine and 10 to get to five-under. He birdied four of the last six holes to shoot a course-record and grab a share of the lead.
Delaet played the back nine first and birdied three of the first six holes. After five straight pars around the turn, he dropped a shot on the third. Delaet birdied the next two holes before a bogey on six left him at minus- nine.
The Saskatchewan native posted birdies on seven and nine to tie Heffernan for the lead after two rounds.
Danny Mijovic, who owned a piece of the first-round lead, struggled to a two- over 73 Friday. That dropped him into a share of eighth place at three-under- par 139. He was joined there by Alan McLean, Chad Spencer, Travis Johnson, Brendan Steele and two-time winner this year, Adam Bland.
Andrew Johnson, who was tied with Mijovic after Thursday's opening round, struggled to a three-over 74 to fall into a tie for 14th at minus-two.
The cut line fell at three-over-par 145 with 68 players moving on to the final two rounds.