Girlfriend accidentally gives phone number

He's right, you know

Never thought lesbians broke up like this

This makes no sense. Wether OP wanted to hurt the ex or spare her, either way she will be hurt, and either way she will be fine. The only conceivable reason I can see for this is an attempt to spare their own pride at having been cheated on, or because they know if they confront her that they are too weak to follow through if she tries to stay together, and wanted the only way they felt she was guaranteed to leave. It's a childish mess of poor planning and emotional immaturity. Break up face to face, always, unless you have reason to doubt your safety or its somehow physically impractical (like a long distance thing,) and just be honest about the breakup. You don't have to be brutal if you don't want but don't lie- be better than this post.