Wow, rough one. Would have to get into the language of the specific policy and more importantly the Driver Exclusion endorsement. Guaranteed HO policy does not cover.
Read the actual driver exclusion, your answer lies within that document. If vague (as it may likely be in this situation), the interpretation will be done by the clams department of the carrier. If they deny, the cyclist can proceed with a UM claim under their own policy and their carrier will come after your insured to recoup their money.
If your client disagrees with his own carrier's denial, he can sue them, agree to arbitrate the dispute and/or file a complaint with the CA Dept of Insurance.
Read the actual driver exclusion, your answer lies within that document. If vague (as it may likely be in this situation), the interpretation will be done by the clams department of the carrier. If they deny, the cyclist can proceed with a UM claim under their own policy and their carrier will come after your insured to recoup their money.
If your client disagrees with his own carrier's denial, he can sue them, agree to arbitrate the dispute and/or file a complaint with the CA Dept of Insurance.