One of my mantras is never use an inflexible finish on a flexible seat. Hard, surface floating finishes like Shellac, lacquer and traditional polyurethane wont flex with the seat as it is used and this could have contributed to its deterioration. But there may be other factors such as UV damage if this chair sat in a location with bright sunlight it could be ruined quickly, old or poorly stored splint can break in that short of time. Or if the edges haven't been filed round or someone is standing on the chair regularly. Weave it once more with a new hank of splint and oil stain it to the desired shade. Use Oil/ turp or Danish Oil and let them know how to protect their seat from damage but tell them this restoration is on the house but if it happens again you will have to assume they are not caring for the seat properly. I have written care instructions for each type of seat to give to the customer when they get their newly restored seat. It acts as a reminder that they have to be kind to their chair if they want it to last.