Very good question on our limits for compassion.

“To love our enemy is impossible. The moment we understand our enemy, we feel compassion towards him/her, and he/she is no longer our enemy.”
Thich Nhat Hanh

Compassion isn’t about allowing or accepting bad or questionable behavior but about feeling peace and calmness as we evaluate a situation and/or a person and can determine whether or not they are who we want to journey with in this life. The two souls you were referring to made each other sick, or depressed, therefore the compassion would come with letting go of each other. Had they wanted the same things, and valued the same things and were both invested in a comparable relationship, the sickness would have been replaced with communication, trust, inter connectivity, and the desire for a future together. Relationships are like a river..too many stones and the water no longer flows, therefore the river is no longer a river.