Trust that your dude is with you because he likes you and you're awesome, not because he's biding his time until your friend takes him back.
Trust that your friend is happy you've found someone you dig, not plotting to sabotage your love.
Likewise, don't grill your boyfriend on what went wrong or insist that he account for his behavior throughout the entire time they dated.
Their relationship is between them; it's not your cautionary tale or your soap opera. It's easier, of course, to have hard-line rules — "exes are never OK" versus "exes are totally fine" — but that's not the world we live in.
We know our backstories will be tangled and intertwined.