Probably the easiest way, if not necessarily the best option, would be to see if the ARV regimen can be swapped. If they're on stavudine, for example, this is not a bad idea since it is such a toxic drug. Otherwise, not sure a biopsy can distinguish? Does crystal meth really cause PN? Isn't it more of a CNS dopaminergic neuron damaging kind of drug?
Also, how else are you treating the neuropathy?
And are you able to treat the meth addiction? Are there any services for that?