I'm going to assume you don't actually have $15,000 in liquid assets, considering the $70k student loans and the mortgage payment. If that's true, then getting the 15k means either saddling yourself with debt, or taking money out of your retirement. And honestly, if I had a child who did this to get me some land, I'd feel very badly about it. Happy, sure, but also bad about what was done to get me that. Worse, when it's done it's done, so if she felt bad, she really couldn't just say "no" because it's a done deal.
Thinking about helping her out is a good thing, by the way. I dig it. So maybe brainstorm and see what else you can do, short of coming up with $15,000. Maybe even just think small. More like this:
Go to the grocery store. Pick up some fresh groceries. Take them over to Mom's. Now, cook a meal for the three of you, right there in the kitchen. Quality time, a meal, and when you're done, leave the groceries. Tell her you'll come back soon to cook up another meal.
This gives you time together, which is priceless. And it means she won't need to buy groceries that week, which helps a little with finances. Maybe it's not 15k. But it is good.