It all went down Saturday night, when Dan -- an asst. coach at St. Mary's Prep in Michigan -- orchestrated a little halftime surprise for his girlfriend Chelsea ... who works with the cheerleading squad.
After shooing away the team mascot, Dan -- who scored the $500,000 grand prize on "BB10" -- dropped to one knee and whipped out a 2-carat diamond ring in front of a massive crowd.
We're told D&C are planning to tie the knot next summer.
But the best part -- St. Mary's won the game 14-7.