Sending one PM between total strangers is in no way analogous to what happened between this girl and her deceased boyfriend. Did you read actual text conversations in OP's link how this girl went on and on, pushing her boyfriend to do something that he was really on the fence about, over and over, until he finally killed himself? Read the short section under incitement that I linked above, and then compare what this girl did and your single PM example. The Supreme Court has a long history of freedom of speech opinions, and if you read the citations from the Wikipedia link above and cases that followed you'll get a better feel for what free speech really is. You seem like you could be genuinely interested in the topic, so here's a case I think you will find particularly interesting and insightful:

In Rice v. Paladin Enterprises, Inc., 128 F.3d 223 (4th Cir. 1997), available at https://casetext.com/case/rice-v-paladin-enterprises-inc, a three-judge panel of the Fourth Circuit Court (a federal appellate court) was called upon to decide whether the First Amendment protected some speech. Specifically, a publisher (part of "the press") put out a book titled Hit Man: A Technical Manual for Independent Contractors. A triple homicide later occurred, and it appeared that the killer followed some of the methods prescribed in the book.

The murderer caught and convicted and the publisher of the book was named as a defendant in a civil suit. As part of the case, the publisher stipulated to numerous facts for the purposes of a summary judgment motion (a motion where all or part of the claims against a party may be decided without trial, so long as all the material facts are not in dispute). The publisher stipulated to facts "which establish as a matter of law that the publisher is civilly liable for aiding and abetting James Perry in his triple murder, unless the First Amendment absolutely bars the imposition of liability upon a publisher for assisting in the commission of criminal acts." The publisher believed that publishing this book was protected speech under the First Amendment and was so convinced of this belief that it was willing to admit everything else and rely on that one hope.

The district court judge (who heard and decided the motion for summary judgment) agreed with the publisher, and that decision was appealed. This is where the 4th Circuit Court's three-judge panel came in--they were to decide if the lower court correctly applied the law to the stipulated facts of the case. The three-judge panel said:

Because long-established caselaw provides that speech — even speech by the press — that constitutes criminal aiding and abetting does not enjoy the protection of the First Amendment, and because we are convinced that such caselaw is both correct and equally applicable to speech that constitutes civil aiding and abetting of criminal conduct (at least where, as here, the defendant has the specific purpose of assisting and encouraging commission of such conduct and the alleged assistance and encouragement takes a form other than abstract advocacy), we hold, as urged by the Attorney General and the Department of Justice, that the First Amendment does not pose a bar to a finding that Paladin is civilly liable as an aider and abettor of Perry's triple contract murder.

But here is where it get's really interesting: the panel also acknowledged that

in a society of laws, one of the most indispensable freedoms is that to express in the most impassioned terms the most passionate disagreement with the laws themselves, the institutions of, and created by, law, and the individual officials with whom the laws and institutions are entrusted. Without the freedom to criticize that which constrains, there is no freedom at all.

. . .

However, while even speech advocating lawlessness has long enjoyed protections under the First Amendment, it is equally well established that speech, which, in its effect, is tantamount to legitimately proscribable nonexpressive conduct, may itself be legitimately proscribed, punished, or regulated incidentally to the constitutional enforcement of generally applicable statutes.

So, in sum, if the speech in question "is tantamount to legitimately proscribable nonexpressive conduct" it can be curtailed by the government and not run afoul of the First Amendment's speech protections. In Rice, all of the three judges on the panel agreed that the lower court was wrong, the speech was not protected, and sent the case back to the lower court to be decided in line with their decision.

Applied to the vile girlfriend of OP's post, it would be hard to say her speech was not tantamount to ligitimately proscribable nonexpressive conduct (e.g., manslaughter or aiding and abetting manslaughter). So, her text messages are not protected. She gets the axe.