I've toyed with answering in this thread a few times over the last couple of days and not done because without seeing a photo of these 'spots' it is hard to say anything.
Why are a few spots of dehydration bothering you so much? Is it bothering the client excessively. She sounds like she has mature hands and nails, which tend to be dry naturally ... They are ridged, which is a sign that her nails are dry ... Dry isn't something to get too upset about is it? It is nothing permanent or deleterious. A bit of surface dehydration is no worse than hair being dry after bleaching.
Anything that disturbs the surface of the nail can cause dry spots to show up. Using a metal tool aggressively to remove cuticle can cause dry areas to form, product removal has to be gentle; Only you know if these things could be the cause or if she just has dry nails.
Solar Oil has an accumulative effect over TIME and if she uses it regularly and daily, then you should find any dryness getting better over time. Keep a watch on the nail condition and if the problem gets worse then discuss what to do with the client then.