If the pecan is still in the shell, could it still cause an allergic reaction if someone allergic to nuts still handles it?

A reaction is possib: There can be a reaction to either the shell or the actual nut. Pecans are encased in a thin shell. As reported by Joyce et al, 2006: 'The relative frequencies of reactions (systemic or localized) to specific tree nuts were reported as follows: walnut, 34%; cashew, 20%; almond, 15%; pecan, 9%; pistachio, 7%; and hazelnut, Brazil nut, macadamia nut, and hickory nut each were less than 5%.'

Highly unlikely.: But if the shells have been contaminated by the nuts in the process and the handler touches his own lips after handling it, there is a theoretical possibility but in general one would not expect any reaction. The report of reaction to peanut dust in the air is something I have yet to witness.