Just to nitpick your terminology, when you say someone “has” Russian debt, that usually means that Russia owes them money - that some payment due from Russia is owned as an asset.
In your scenario, if you owe money to some sanctioned Russian entity -
…well, the challenges are not really financial. You still have the same liability, you’re not going to get the windfall of the debt disappearing. It’s more a questions of legal compliance with whatever sanctions are in place. Ultimately, your own government would control what happens to any repayment, along the lines of what’s described here:
In the U.S., this is what happened in WWI and WWII:
See also a prior thread on this topic:
What happens to economic debt between belligerent nations?
In terms of major public debt, Russia has obviously tended to be a borrower rather than a lender from the West. Substantial Russian loans would often be politically motivated. According to this article (2019), Russia’s biggest debtors were Belarus, Ukraine, Venezuela, Cuba, Bangladesh. I guess any repayment from the second of those will be taken rather than given.