Being South Indian this is very important question. Please answer clearly and if you have to ask victims parents, Please do so.
" If Victim's Parents used to do an annual sacrifice or offering, could be victims Grand Parents Used to do it from whom victim's parents inherited house / land, and then victim's parents stopped doing that offering for some / whatever reason"
was the victim interested in Shaktism and visiting places?
South Indians have family, village deities and then most do offering to a specific deity once or twice a year. all deities are different and they react differently under different circumstances.
before we blame any relative or anyone else, we need to check family history.
soothsayer who can only tell the problem and can not help is not a very reliable authority, when victim/family spoke with the hindu got through that lady, did god introduced him/herself? when shakti comes on a person it is always introduced. which god that was Kali, Kateri, Kal Bharav or some other? Now god himself / herself manifested and did not gave a solution which is difficult to comprehend, is it that hindu god accepted that whatever Jin that is, is more powerful then him /herself?
victim's parents and grand parents worship to which deity daily and annually and why stopped.
Kateri, Bhairav, Sagani, Sangalini, kurupan and some others can work in both positive and negative ways. answer the questions and I will try to suggest.
If family was a Muslim then no matter what jin, bhoot preet etc it is could be dealt easily without going into details. now trying to help remotely will need some basic info and then may be specific details.