Translation:They should tell the boss about it.
"They must" does not express the conditional mood there is in "faudrait". The best translation is really "they should". Alternatively, you can use "they would need/have to".
You missed the translation of "en", which stands for "de + something mentioned before", and which translates to "about it".
So I put "They would have to talk about it with the boss. DuoLingo said "They would have to talk with the boss about it." I'm sorry, but I don't see the difference! I'm sure grammarians will have a complex answer why my answer was wrong; but it really bothers me when things like this come up on Duo. Thanks for letting me vent!