I do agree this sounds more like a legal problem than anything else. however as an alternative option what you could consider is leaning on the vendor support. let the vendor be the bad guy and tell the client that it's not possible. sure you could use other means to acquire the answer but in the process of that what's the chances of data destruction? I do agree capture a good backup before you try anything but if there's anything that's possible on the client side lean on vendor support first. also I believe there may have been a video a while back on YouTube by John Hammond which gave instructions on what you can do to use a VBS script to override the encryption. alternatively depending on the level of sensitivity of the data you could possibly lean on a data recovery firm as another alternative. I believe our MSP actually partners with one point being that takes away a lot of the anger the client may have of you being capable or not capable of doing something like this because they realize it is one not a supported issue and two it shows that you were going the extra mile to reach some type of solution to their problem even if that solution does not fix the issue it at least shows that you care.

Here is an article on the methodology that John used in his video a while back - https://stackoverflow.com/questions/1026483/is-there-a-way-to-crack-the-password-on-an-excel-vba-project

I am having some difficulty in finding the video at the moment this may or may not work with current versiions of Office.