Translation:The girl went with the boy to the party.
This isn't really passive in Danish either - the sentence is crappy. Either the 's' at the end of the verb (which is making the passive) should be removed to match the English sentence, or the preposition should be changed to 'av' to express 'The girl was followed by the boy'...which would be the matching English sentence.
you've made several comments using the Swedish (and I think also Norwegian, but I'm not sure) 'av'. As far as I'm aware this is not a word in Danish - the correct spelling is 'af'
The s at the end isn't necessarily passive. In this case it is reciprocal - see here: https://forum.duolingo.com/comment/28260240/Differentiating-S-Verbs-Deponents-Reciprocal-Passive . This also explains that other participants of reciprocal verbs can be added with the preposition 'med'.
Here is "følges med" in Den danske ordbog https://ordnet.dk/ddo/ordbog?subentry_id=59002882&def_id=21025769&query=s%C3%A6tte&mpage=3 . The definition is "gå sammen med en person; gøre en person følgeskab"
tl;dr the Danish is correct
This Danish course is full of errors, and you are completely right!
The Danish sentence is wrong, and there are two alternatives to fix it:
'Pigen fulgtes AV drengen' --> The girl were followed by the boy. This is the passive alternative.
'Pigen fulgte med drengen' --> The girl followed the boy. In this case, there's no passive.
The current Danish sentence is a mix of the 2 alternatives here, and it doesn't match the English text.
No, there's no such rule (and btw the sentence in this exercise has a Direct Object).
'Pigen fulgte med drengen' --> The girl followed the boy. (Passive form not used, and 'the boy' is D.O.)
'Pigen fulgtes AV drengen' --> The girl was followed by the boy. (Passive form used, thus "turning the sentence around", and the boy becomes controlled by the preposition rather than a classic D.O., i.e. would typically be dative (e.g. German) or genitive (e.g. Russian) rather than taking the accusative D.O. form.)
The Danish sentence is a mix (not a good sentence), and the translation doesn't match, really.