Mrs Tina was talking to Mr John. The guy said something funny and then the narrator wrote:
Conscientiously, Mrs Tina joined in his laughter.
Does it mean that Mrs Tina laughed exactly in the same way as Mr John?
Not a teacher only a native.
Not exactly. I would say in this context, 'conscientiously', meant that Mrs. Tina made a descision to join in the laughter. Mrs. Tina thought about laughing before doing so, as opposed to laughing spontaneously.