Most Helpful Girl
When one of them aren’t happy being with their partner and if the person doesn’t have feelings for their partner. That’s when the relationship should end. It wouldn’t be fair on the other partner if they stayed in a relationship with their partner.1
Most Helpful Guy
There's never a "good" time, but some thoughts would be:
Don't wait until you resent her presence
If you have ANY feelings for her it's not too late.
If you realise something's wrong, talk about it - to her, not your drinking/poker buddies.
From your question though, you already have. You just didn't tell her yet.0