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