It would depend on how your client presents with seborrhoea as symptoms vary widely. Some people also have associated conditions which you may have to include in your consideration too.
For example, a client of mine diagnosed with seborrhoea (seborrheic dermatitis/eczema) presented with swelling, particularly around the eyes and centre of the face. As part of my consultation, I took a history of her condition and read her doctor's notes which I used to develop a treatment plan for her.
In my case, after tailoring her facial to the specifics of her condition, I showed my client how to perform a cold compress and demonstrated how she would use the skincare items I recommended she use which was a rose mist and a moisturising cream from the range I was using.
I would recommend you talk to your client, find out if he/she is working with a medical practitioner, take a history, formulate a plan and work out a regimen that suits him/her.