And I have no idea what it is, but the concept of 'you pay nothing each month' seems like a scam. Is it?
Edit: After reading, it seems like something for your property you're already in, not a new property. So you owe $200k, the bank gives you $200k and says you don't owe us anything for X years, but we're going to charge you interest and you have to continue to pay the property taxes. So you then invest the $200k in the market hoping to get more than the interest you paid on your original mortgage, so that in X years when your balance of $400k is due you actually made $550k in the market. Right??
As your figured out and others have noted, it's for people who are "house rich" and "cash poor", that is, sitting on equity. She won't qualify for one unless she pays all cash.
A good question for your mom is, "what are you trying to accomplish?" What are the financial aspects of her situation? What are her sources of income (pension income, SSI, other), overall savings (cash, investment accounts), withdrawal rate from various accounts. What are her expenses? Has she sold a house and looking for a replacement?
You don't need to share with us, but understanding where she is and what she wants to accomplish will give you a better chance to help her make an informed decision.
BTW, a reverse mortgage was the absolute best thing my grandparents did in their later years. My grandmother wouldn't consider moving and it enabled them to stay in her home of 80+ years until her final days.