A friend had chronic demyelinating polyneuropathy (cidp), then got chronic lymphocytic leukemia 2 years later. Is cidp paraneoplastic syndrome sometimes?

Not likely. Chronic inflammatory demyelinating polyradiculopathy (cidp) is almost always an autoimmune condition incited by some antecedent infection--c. Jejuni, ebv, cmv, or another infection. Antibodies produced against the virus/ bacteria/ pathogen cross-react with the myelin sheath on the nerve roots, first causing guillain-barre (aidp), then later cidp. It's exceedingly rare that cancer could cause this.
It wasn't CIDP. Classic cidp is a unique autoimmune peripheral nerve disorder with particular characteristics and criteria to make the diagnosis. Yes, peripheral nerve problems can be paraneoplastic, but a careful expert analysis can usually sort out the peculiarities, and look for other explanations. A well done EMG can usually point the way. To be fair, maybe two disorders were present.
Can be. Yes, it can be. That is why it is important not to simply accept the name attached to a clinical problem, but to look for underlying causes.