I know that sometimes people will do this and it's discouraged by the airlines.

Here's the situation. I bought my in-laws a plane ticket on delta with miles. They have a layover in SLC. My MIL decided she would like to fly to SLC and stay there a few days. I tried to change her flight or cancel it says I can't. So what would happen if she just got off in SLC and didn't get on the next flight? Would it negatively impact her or I or my FIL (who will be travelling on) in any way?