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.
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.
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. ]
Ahhh okay, so his old first girlfriend. That makes sense, thanks.
"Started dating his first girlfriend again" might have been clearer.