How long does this ability need to be available in the future?
Do they plan to use MFG/Pro as the method for retrieving data? Or do they intend to "just run some ad-hoc queries"?
How much are they willing to invest in this archive process? (Judging from your past posts I'm guessing roughly $0.)
You are already on an ancient, obsolete and unsupported release of Progress. As time goes by the ability to retrieve data from it will become increasingly tenuous. Your best protection from that over time is to dump the data into a platform-neutral format. But if you do that you will not be using MFG/Pro to extract anything from it -- my guess is that that is not what they have in mind.
Assuming that the preferred solution involves using MFG/Pro to access the data and report on whatever future questions might arise you need to ensure that you preserve:
1) All of the databases -- take an OFFLINE backup after truncating the bi file. This will maximize your ability to restore in the far future.
1a) I would also dump the data, the schema and all of the SQL grants & views etc. I would do both binary and ascii dumps to maximize flexibility in the future.
2) The Progress binaries. The original Progress install media and activation codes would be pretty handy to have too.
3) All of the code and all of the modifications to MFG/Pro
4) System level stuff like user logins, printer definitions etc, etc might be needed too.
5) Documentation. You need to document all of that stuff that you haven't been documenting before now. Especially startup, shutdown and recovery procedures.
Personally, I would put all of this stuff in long term storage with the least likelihood of hardware failure or long-term incompatibility. Possibly something like Amazon's "Glacier" offering.
In the future it may not be easy to obtain a machine with a compatible OS to run the Progress binaries. So you would be smart to keep the OS preserved somewhere too. And if you are using tapes for backups you should make sure that you have a compatible tape drive with which to read them. Actually you should probably have 3 or 4 spare tape drives and then pray that the tapes themselves don't fade away...
You can not be confident that your solution is workable until you test it. So you should build a new server from scratch, install progress and MFG/Pro on it and then restore the db backups. If you can access the data then you at least have a chance that you have done it correctly.