@oneservant All that matters in the end is who is on title and who is the lender.
Here’s the best case scenario i can see with what I know.
Mom’s friend is the lender/mortgage holder
(Look up Olympia Trust or similar services to set up private mortgages legally)
Your SO owns the property and is the only person on title. ( use real estate lawyer )
Your mom is a tenant and signs a lease.
In this scenario, SO takes all the responsibility of owning the house, taxes, utility payments, major repairs. And paying the lender every month. (If you miss payments the property will go back to the lender. )
The only problem is if mom stops paying rent what will SO do? Obviously it’s mom you can’t kick her out. But when you do sell in three years just make sure that all the mortgage payments and utilities, “holding costs” that SO puts in are added to the price as an expense, if the real estate climate allows for it in 3 years, sell at the right price and do your best to break even with your holding costs.
All the best.