Other mistake: During the final scene on the cliffs of Chateau d'If, the Count, Mercedes, and Albert are standing on the edge of the cliffs and you can clearly see aeroplane contrails cutting through one of the clouds. When they cut back to the sky, they are gone. (02:05:55)
Other mistake: After the knife fight and the Captain giving Edmond the name Zatarra, the scene cuts to the ship entering a port and the words "three months later" are shown on the screen. By the movie timeline it should have said "Three Years Later." (01:05:20)
Other mistake: In the dancing scene that takes place toward the end of the film at the home of Fern and Mercedes . When the Count of Monte Cristo asked Mercedes to dance with him, there is a close up of Mercedes' face where you can see that she is wearing contact lenses. There a few more scenes with her where it is very noticeable. (01:31:55)
Other mistake: In the first scene at Marseilles, the shipping company's sign reads "Morell & Co. Shipping", but the owner's name is spelled "Morrel". French spelling didn't vary at the time, since the French Academy had by then been in place for some 200 years. And even if a businessman might not spell everything correctly every time, he would at least know how to spell his own name right and would make sure it is so on such a commercial sign.
Answer: I believe it's just because there is a dirty smelly man that she doesn't recognise visiting her ailing grandfather, she just wants to keep an eye on things.