A very low fat diet is key to managing the condition. Hopefully that is in place?
Commiphora and Curcuma have both shown efficacy in lowering serum triglycerides (and thus chylomicrons). They are combined in the same formula, Natural Path's Modified Xue Fu Zhu Yu Tang, available from A Time to Heal in the US. The formula is used for peripheral tumors, so it is a good fit.
Lastly, to help heal the current lesion and allow it to epithelialize, you can mix Natural Path's Xian Fang Huo Ming Yin (Angelica and Mastic Combination) in with a basic wound healing salve (like Calendula, Paw Paw ointment, etc). Use about 10g per 3 oz of salve and mix in thoroughly. Apply once daily, liberally, holding the ointment in contact with the wound using a telfa non stick pad and some vetrap or equivalent.
Hopefully after a couple of weeks, you see some healing progressing, shrinkage of the xanthoma, and lowering of the lipidemia. Let us know how it goes!