No, this is absolutely not clear.
Does your user trust data in BW or not? If they do, then just compare data in BW cube and BPC with plenty of options to do this.
If your user doesn't trust data in BW then question is if your BW and BPC are on same box or on different?
If they are on same box then most probably it has an RFC to ECC and you can use something to get data from ECC to compare. If they are on different boxes then main question is: does BPC box has RFC connection to ECC box?
See how many additional questions your issue brings?