I should add that the C-corp became insolvent (which is why the bank discharged a portion of the debt). However, the individual taxpayer is not personally insolvent. Does that mean the 1090-C income will be taxable? Can the insolvency exclusion still apply?

Taxpayer assumed personal responsibility for the loan, so I would see it as a 1040 income item.
Since the corporation was insolvent, has it closed? Do you have a stock loss as at least a partial offset?

As a side note, I THINK that might increase his Basis in the corporate stock (I would need to research it to be sure though).

Corp did not close and he did not loose the asset. However the asset has lost the majority of it's value.

Thanks for the response George4Tacks! Will you please see the additional info shared above and advise

I do not believe that canceled debt is taxable to a guarantor.  The reason being that, the guarantor would have recourse against the original debtor, and therefore the canceled debt would result in a deductible expense (bad debt) and therefore would not be included in income - see IRC 108(e)(2).

https://www.law.cornell.edu/uscode/text/26/108

I also believe that it would be incorrect to issue a 1099-C to a guarantor as per 1.6050P-1(d)(7)

https://www.law.cornell.edu/cfr/text/26/1.6050P-1

Substantial authority is hard to come by, but this letter from the IRS references the relevant case law:

https://www.irs.gov/pub/irs-wd/02-0024.pdf