Reviving this one with an update. Much discussion went on amongst sign shop (me), the tenant, and the landlord. We worked it out so the landlord is covering all the costs associated with the sign divider, though he isn't happy about it. Apparently there's nothing in his tenant leases about who gets which position on the directory sign, which led to this whole mess. At least it's not costing me anything and was a good lesson on how to handle multi-tenant signs in the future.
And fast forward to the future...today. I was asked by a building tenant to add a new plaque for them to a lobby directory sign which is (mostly) in numerical order by suite #. The plaques are all attached with tape and there's no space for the new suite. You would think something like would be simple, but no. I contacted the building manager to find out if they want me to fix the whole sign - it's looking pretty run down with different font sizes and alignments and no easy way to add a new plaque where it belongs, and someone's been stealing standoff caps along the bottom. That's going to be on the building manager, not the tenant (my customer).