On Saturday night in Shawinigan, Beaulieu and his mighty Sea Dogs face the Knights in both teams' Memorial Cup opener.
The old friend is now the foe.
"It means a lot because it's my hometown team," the Strathroy native said. "I grew up with the Knights and it's going to be special to play them because there are still a lot of people in that organization who are like family to me.
"It was nice to see, but this week, that's over."
He's not rooting for them anymore.
His Sea Dogs, looking for a second straight Cup crown, plan to come out flying.
It's interesting because Beaulieu, if things turned differently, might have been flying out to Quebec with the Knights on Wednesday.
"They were my team," he said. "I spent a lot of time watching and listening to how they operate."
His dad Jacques, now in charge of the Sarnia Sting, was a London assistant in those glory days.
But he ended his first stint in 2006 to coach in Saint John and take the next big step in his hockey career.
His son did, too.
Had the Beaulieus stayed put, Nathan would've been exposed to the OHL draft, the Hunters always look after their own, and how good would the strong London blue line look now with him, an NHL first rounder and world junior defender, on board?
"I knew in the summer the Knights might make it (to Shawinigan," Nathan said. "I worked out with (fellow Montreal Canadiens first-rounder) Jarred Tinordi (at Total Package Hockey) and he thought they were going to be a pretty good team."
Tinordi kept tabs on the Sea Dogs, too.
"Everyone knew they were going to be strong," he said. "They won the Cup last year and were getting most of their guys back. We were roommates in Montreal and we obviously both like to talk about hockey. I told him we were pretty young but I thought we were going to be good, I just didn't know how good.
"As it turned out, pretty good."
Nathan Beaulieu just took it for granted his dad Jacques and the Knights coaching staff would talk before this tournament about the upcoming game.
"But I'm not worried about that," he said.
In fact, London GM and head coach Mark Hunter said there has been no dialogue with Jacques this past week.
Saint John is a formidable first opponent.
They only lost one playoff game - in overtime - and never trailed in the league final.
Born in December, Beaulieu is still the youngest defenceman on the team. He turns 20 later this year.
"We have more experience," Beaulieu said. "We went through and won last year (in Mississauga), and this season, I wouldn't say we felt pressure to win, but we knew what we had to do. We lost some key guys who were a big part of winning it, but other guys have stepped up."
He's grateful Tinordi will be at the tournament. Together, they can divvy up the Habs-related questions.
"I think he gets it more than I do," Tinordi said, "and that's because he's there playing in the Quebec league. It's good to have someone you know going through the same things.
"But this week, the focus will be on helping our teams and trying to win the Cup."
All those ties are severed for the sake of sport.
(Strathroy native Nathan Beaulieu, a Saint John Sea Dogs star defenceman and Montreal Canadiens first rounder, has a Memorial Cup title and a Canadian world junior bronze medal to his credit).
2008-09: 49 games, 2 goals, 10 points, minus-4, 14 penalty minutes. Playoffs: 4 games, 0 points.
2009-10: 66 games, 12 goals, 45 points, plus-43 40 penalty minutes. Playoffs: 21 games, 4 goals, 16 points.
2010-11: 65 games, 12 goals, 45 points, plus-44, 52 penalty minutes. Playoffs: 19 games, 4 goals, 17 points.
2011-12: 53 games, 11 goals, 52 points, plus-41, 100 penalty minutes. Playoffs: 17 games, 4 goals, 15 points.