朋友結婚要我做見證人，但她問我要身份證副本先，話由佢交比律師先,因涉及個人資料,想請教吓,難道唔係見証果日,由我交比律師嗎?

Both (your friend and you) are correct.

The request is due to how the marriage certificates are prepared.

Witness information is preprinted on the certificate. When the marriage ceremony is conducted by the Immigration Department, the certificate is prepared onsite and printed just before the ceremony. However, when the marriage ceremony is conducted by a lawyer, it usually means the ceremony is conducted offsite and the lawyer will have no access to a printer to print the certificate.

That's why the lawyer asks for a copy for your ID, so that the certificate can be prepared in advance and ready to be used during the ceremony.

不要交給任何人，待証婚儀式進行時，交給婚姻註冊署登記，即時當面取回。