I know I'm going to step on some feelings here. Sorry in advance.
Legally, a finder of lost valuables in my state is entitled to a 15-20% "finder's fee", regardless of what is found. The courts here have upheld that amount whether the "cache" is recovered bank funds or intentionally cached objects. So regardless of your relationship with your friend, start with that amount.
If your friend also owns the property where the cache is located, you need to consider the space to be covered, and how quickly you can effectively cover it.
Unless your friend can document the person who actually cached the funds, and you can document they had such funds, I'd pass. The 5 W's still apply: Who, What, When, Where, Why. Unless you can document at least 4 of the 5, this comes under the classification of legend or heresay rather than fact. Wait until you have at least a name before commiting to a lengthy project, and do all possible research on that person first.
In this case it sounds as if some interviews with your friend's family members might be in order. Time is money. In even researching the story, you are investing your time and expertise in interpreting history, as well as your expertise in using a metal detector.
And as CWnut notes, there's still the chance that the family of the deceased could make a claim against anything recovered if you or your (land owner) friend talks. Just some realities of the business, and I hope I haven't offended too many people here.