[jvzr]

I understood it as: he married and divorced someone else, then started dating his first ever love after GP said that. He probably realized the depth of his relation with his first love.

[StavrosK]

Hm, that makes sense but it's also a bit of a stretch from the GP's comment, I especially don't understand why the wife doesn't count as a girlfriend.

[vmilner]

She does - but she wasn't the first. [ First girlfriend = Alice (say). Breaks up with Alice. Dates more girlfriends. Marries one. Divorces. Dates Alice again. ]

[StavrosK]

Ahhh okay, so his old first girlfriend. That makes sense, thanks.

"Started dating his first girlfriend again" might have been clearer.