PIP asking for passport ID.

Hi Team
I am supporting someone to apply for PIP, they made the phone call to obtain Application Form, & had text message to say form had been sent.
The following day they had a random text from the DWP to say to get back in touch as they needed to verify their ID. When they contacted they said they needed them to post their passport to them.
I have never heard of this before & find it very strange for the PIP process. The Applicant, who was reluctant to apply anyway, is very worried about some sort of scam. Plus I don't think anyone really wants to post their passport off.
Although it was the same telephone number he used to ask for the form.
I would be very grateful of any advice you can give on this matter & wonder if any other members have had a similar experience. ?
I was also wondering if there is an alternative, more reasonable, ID check that DWP could use.
This has made the Applicant very nervous, they have not actioned the request as yet.
Thank you.

I've been getting DLA/PIP for 30 years yet now have had a letter from them asking for ID without any reason mentioned in the letter which appears genuine since it provides an SAE for the right address. Just hugely weird - I mean if they don't know who I am after all this time they have a serious problem I doubt!!