I disagree with the views that you have expressed. It is not the original education certificate or degree certificate that the new company is asking for. A relieving certificate or experience certificate is typically printed on a company's letterhead and issued to an outgoing employee. It states that a particular individual was employed with them and got relieved due to resignation, etc. I don't believe there is any significant confidentiality or importance attached to this.
I am unsure why this company is requesting the original certificate. If the candidate wishes, they can keep a photocopy with them and hand over the original letter. What could they possibly do with this certificate? Even if they do not return the ORIGINAL relieving letter or experience letter, so be it.
When the individual moves to the next company, if that company requires a copy of the ORIGINAL certificate, they can always explain that they had submitted the original to their previous employer and provide the copies they have in hand.
As long as the candidate is genuine and straightforward, I do not believe this will cause any harm.
Regards,
V. Balaji