An unapparent cause.
It pretty obviously requires further information.
If Raynaud’s, this is one of the few situations where biofeedback has shown documented positive results (most other claims are woo.)
Please have your friend get this checked out soon, especially if they have any apparently unrelated symptoms, like breathlessness or changes to skin texture or coloring. If any of these are present, a referral to a rheumatologist is recommended. My wife had diffuse scleroderma which started out with Reynaud’s syndrome.
The internets have taught me that this is never the cause.