I have some questions for you:
Awaiting your reply.
Sea buckthorn should be safe for use in cats, but discontinue if any vomiting or diarrhea occurs. I did not come across any studies that determine how effective it is, but it is an antiinflammatory.
Stomatitis is believed to be caused by calicivirus which has a tendency to stay in the body and cause problems when the immune system isn't working as well as it should. A product called Viralys or L-lysine is an excellent antiviral and will help prevent the stomatitis from getting worse. Dental care will also be very important in your cat's case.
Let me know if you have further questions or if I can be of further assistance.