The correct answer in this case is no you wouldn't - it is and should be entirely up to the victim.
If the 'victim' didn't want the matter pursued, what would be the purpose of that person now reporting the matter to the elders?
There should be a stated policy to the congregation that any sexual assault or abuse reported to the elders, will be reported to authorities. It is then the 'victim's' decision if he/she wishes to report the matter to elders, knowing in advance the elders will report the incident to authorities.
When there is a crime committed, it is not an option for the victim whether or not the matter is investigated and the purp is prosecuted. The only decision a victim may be able to make is whether or not the crime is reported, to anyone. Once reported to elders, then responsibility is transferred to elders if the victim is not willing to report to authorities themself.