The loans and taxable scholarships/value of living with grandmother are more than student's earned income so student isn't going to have earned income greater than 50% of support.

This is what I was thinking of doing.  Last known whereabouts of missing parent was in a large metro area.  I have the median household income for that area as of mid-2021.  I was going to use that, double it, and assume a single marital status, all being disclosed in an attachment to the return.  I'm figuring I'd rather be conservative and pay too much than have IRS come down on student for not paying enough.  The student doesn't completely understand this but would rather not have to deal with an IRS notice.  Plus my time involved with responding to them will be more than the extra tax paid.

I appreciate your response Rick!