Won't hold up, unless he mentioned it later. The ex doesn't specifically state breaking her window in that text so it isn't good enough evidence. He could always play it off like he was expecting to get proposed to
56, no convictions are needed in civil court. Depending on the judge, their relationship history and that text should be enough to at least award OP enough to replace the window.
Even if she has no intention on taking the ex to court she should still report the text along her smashed window to the police. This way if anything further happens she has a record of it, it would help later when the ex does something else. It also helps in the event she needs a restraining order to have a record of previous issues such as this one.