This is a simple (non wiccan) spell that may help someone realize they're in a toxic relationship.
You need two taper candles with sturdy holders, string, a small handful of rocks and a safe place to be using fire.
Step 1: mark the candles so they symbolize each person in the relationship.
Step 2: tie the stones to the candles so that they hang between them. Repeat this every inch or so up the candles .
Step 3: light the candles.

As the candles burn the stones should drop, symbolizing and issue in the relationship building a wall between the two individuals.

A thing to note, this should never be used out of jealousy. Only as a last resort for someone you're concerned for. As the issues come up the couple may learn to overcome them and come out stronger, if that's the case the relationship was probably not as unstable as perceived.