Jon — who also shares twins Mady and Cara, as well as 18-year-old sextuplets Alexis, Aaden, Leah and Joel with Kate — wanted to make their big day something truly special and "threw a huge BBQ" for family and friends.
"Jon and the kids arrived at the graduation in Collin’s brand new convertible Mustang," an insider revealed. "They were all smiles and ready to graduate. Collin and Hannah looked like best friends."
However, despite having been told Kate would be coming, Jon was reportedly shocked she actually made the decision to show up.
"Jon was surprised that Kate had decided to come just because she has been absent for a lot of their lives," the insider continued. "So he wasn’t expecting that she’d show up to graduation."
Making matters even more complicated, Jon's ex-girlfriend, Colleen — who he has remained friends with and who has continued to be close with Hannah and Collin — also attended the ceremony and "surprisingly sat directly behind Kate."
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"It was a bit intense for Jon having both his ex-girlfriend and ex-wife there, but everyone was civil and the kids were the number one priority," the insider noted, before clarifying that Jon and Kate did not speak or otherwise interact "at all" throughout the event.
"She was sat at the opposite end of the place and they didn’t have to see each other," they continued. "They didn’t even look at one another."
And while Kate has been attempting to rekindle a mother-daughter relationship with Hannah, as OK! previously reported, Jon admitted he's been less successful trying to mend broken bonds with his other four sextuplets, who he claimed refuse to respond to his attempts to reach out after Jon and Kate's bitter, 14-year custody battle.
"I will never give up trying to talk to my children. I’m hoping with social media that maybe they might reach out to me at some point," he said earlier this year. "I've tried for years for any interaction with any of my other kids, but nothing yet."
The insider confirmed to The Sun that Kate attended Hannah and Collin's graduation.