There is nothing unusual in skin thickening and unevenness after an operation, particularly around the region of the armpit. However, pain is truly unusual. It is difficult to give you advice without knowing how much time has passed, and without looking at the appearance of the lesion.
If the operation is less than 3 months old, it may be best to wait a bit. If more than 9 months have passed, and the pain is significant, she may be developing some local complication in the skin or in the underlying nerves. This is never serious, but can be troublesome.
Before undergoing reoperation you should consult a senior surgeon in a good hospital.
I do not know if the problem will disappear with ointments, but it may well disappear if given enough time.
Sebaceous cysts do not recur after operation, unless some part of the cyst is left behind. This may occur if the cyst was infected at the time of surgery.