That's a good question! I can only assume they had lost touch over the years. Bonnie was more Carney's friend, I guess? But Betsy and Bonnie reconnected during the events of Carney's House Party, so... hm.
But anyway, I don't recall the Betsy-Tacy Companion mentioning any such visit while Maud was actually in Europe, so I guess it really didn't happen. I wonder.
In the series Bonnie's family had come back to the Twin Cities, so she wasn't in Paris. In real life I think they had never been in Paris at all, definitely not during the specified time, and I think "Bonnie" was one of "Betsy's" sorority sisters.