Once again, an astounding level on ignorance on here!
It absolutely IS a safeguarding issue.
I've been a teacher for 20 years and things have really tightened up in this area over that time.
People going on about "age of consent" have NO idea about the nature of authority in relationships and how that can be abused to groom and manipulate.
Once you have been in a position of trust over a pupil, that dynamic and power imbalance never goes away.
You cannot, as a teacher, contact even ex pupils on social media. In our school it was never, some schools have eg a five year rule.
This teacher has massively breached safeguarding which calls all of her professional judgments into question.
She will be in no doubt about this - safeguarding training has to be regularly refreshed. She is either very ignorant or doesn't care about the rules.
School need to know this, they will not take it lightly and dismiss it as "one message" it's definitely gross misconduct and anyone saying otherwise is either basing it on their own experiences from 20+ years ago or has no idea about these situations are viewed these days!