"They play along."
is Au Bord correct, we think it means at the edge In UK english play along means sort of co-operate, not arguing for the moment at least but not definitely irrevocably commited. The guide said there was a really good restaurant a short walk away and they decided to play along for now.
I have never heard "jouent au bord" as a translation for the English expression "play along". I have only heard it in the context of someone playing on the edge of the lake, the field, etc... I don't think these two sentences go together the way Duolingo is trying to make them go together. Maybe Sitesurf or Remy can enlighten us a little bit.