Must the Friday prayer that was performed outside the mosque, without the congregants inside being seen, be repeated?

The questioner should have arrived early for Jumu'ah prayer and sat as close to the Imam as possible. If he observed any transgressions, he should have discouraged them. His prayer behind the Imam, as described, is invalid according to those who stipulate seeing the Imam or someone who sees him (the Hanbalis and Shafi'is). As for the Malikis, they stipulate the connection of the rows or a small mosque. Their relied-upon opinion is the validity of the prayer, but with strong disfavor, if these conditions are not met. Therefore, the Mufti (definition) advises repeating that prayer as Dhuhr.