Since there are very good reasons to be suspicious of the request, you and the plan's fiduciaries need to protect the plan from potential fraud. Otherwise, if the request is fraudulent, then there will be a lot of finger-pointing (and potentially litigation) about who will make the plan and participant whole.
If there is a concern about inviting the individual into your office or the client's office, consider choosing a public, safe place to meet. This could be at a bank or even at a police station, depending upon the level of concern. The purpose is to arrange for a notary or plan representative to validate the individual's identity. Ideally, someone who could recognize the participant could be available.
If the level of concern is at the level that the plan fiduciaries are comfortable just having an election form notarized without their being present, then consider listing on the document being notarized specific items for the notary to confirm was presented at the signing. This could be at least one item with a picture (like a driver's license), and any additional documents that would acceptable like an original Social Security Card, Medicare ID or something similar.
Ultimately, it is the plan fiduciaries call on how to approach the issue, but you need to make it abundantly clear to them that you believe additional steps need to be taken to confirm the participant's identity.
We have not dealt with this particular set of circumstances, but have had a couple of incidents where we expressed our concerns to the plan fiduciaries and worked with them to document the participant's identity to their satisfaction.
These are just some thoughts and is not advice to anyone.