Sell the SFR as-is and use the proceeds to fix up the vacant unit in the duplex.
Sell both and move into a place that doesn't need work and fits their budget.
Getting a loan would be the other option. Such as doing a cash out refi on one of the two properties. Easier and maybe a higher LTV on the duplex they're living in. Can they qualify? That is, is their credit good enough and do they have income?
Some other source of a loan.
Sell something, such as cars or personal possessions.
Do a shared equity deal with a fix and flipper. That is, agree with a fix and flipper that the fix and flipper will fix up the place at their expense, sell it, and then the fix and flipper will split the profits with your parents.
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