I think it can go both ways, maybe depends on the wavelength of the field, or polarization, to either permit or block certain wavelengths from passing through....Magneto used to use his EM to block telepathy (it's how he was able to surprise attack everyone at Illyana's funeral in UXM 304)...not to mention the way he hampered long range telepathy by messing with the earth's EM field....but it can also amplify, and that's basically how Cerebro/a works.
I think by chance, either in this instance or just Lorna's natural default magnetic resonance, her field proved to be highly compatible with whatever wavelength/frequency Jean was emitting and Jean picked up on it.