I accidentally found such a text message on my boyfriend’s phone. Is it his ex-girlfriend? Does this message mean anything? What kind of girl could this woman be?

This shows that your boyfriend is reluctant to let go of his ex-girlfriend, but he admits that they can no longer be together, and his ex-girlfriend didn't say anything nice to him! So you don’t need to specifically ask your boyfriend about this. After all, it’s in the past. You can observe his attitude towards you together with him. If it doesn’t work, break up decisively to avoid hurting yourself!