If it was a friend's girlfriend - never. If she was in a long term and he was a cool enough guy and faithful to her (didn't diserve it) - never. If things were really going bad, he was ignoring her, and they were both breaking up via cheating - don't think I would unless it was just a one nighter, girls who date guys like that tend to go right back to em and then you just get pulled into their drama. If they weren't serious, it had only been a few weeks, and things never got serious or were pretty much at the fizzle out point - probably if I like her enough. For the most part though I like a girl to at least have some downtime before me. When women don't get downtime IMO its not healthy because if it happens too much it really steals from their sense of identity, they don't know themselves, and it messes with their independence and free thought in general in ways that aren't healthy for her and aren't healthy a lot of times for the guy who picks her right up off the rebound.
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The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.