I would take “eat chalk” literally here; “chalk” probably as in sticks of chalk, for writing on a blackboard.
The humour lies in Phoebe’s assumption that her own very unusual (though characteristic) experience – a boyfriend who eats chalk – might have been repeated in Monica’s case.
That’s an interesting question. In the German version of Little Red Riding Hood the wolf eats chalk to make his voice softer in order to convince the lambs to open the door for him so that he can eat them. So ‘to eat chalk’ my be used to say ‘don’t talk gutter-mouthed’.
There are quite few “Big Bad Wolves” in fairy tales, so I think Ralf might be on to something there. However, I also think Ralf might have been thinking of a different fairy tale – the one with the wolf and the seven little goats/kids (not Red Riding Hood).
I believe there is an expression in German about “eating chalk” which may stem from the fairy tale about this wolf and the little goats. Though I’ve never heard a similar fixed expression used in the US, I think the reference to eating chalk (in the Friends script) may well have been a reference to the wolf in the fairy tale. If so, the intended meaning could also have been along the same lines as the idea in the fairy tale: “Is he a wolf in disguise?” or “(If he eats chalk) he’s not what he pretends to be.” Something like that. I don’t think it’s possible to be sure, though.
Of course, there is a fixed expression in English about a “wolf in sheep’s clothing”. Who knows – maybe the chalk reference was meant to suggest “a wolf in sheep’s clothing” in a different way than usual.
.
.