Knowing a lot of you will vehemently disagree with this, here's the truth -
I'd tell them that I've now managed 20 years without cutting myself, but I do understand. I'd make sure their tetanus jab id's up to date, and then talk to them about ways to do it as safely as possible {antiseptic, keep it shallow, use a clean blade, cover with a dressing etc)*. I would offer them to come and talk to me if they need to, or just need company. Support, not nag.
Because asking them too stop, making them promise to stop etc won't work.it just adds to the pressure.
And Tbh mental health services don't consider it a crisis as they're so overstretched.
And to put it bluntly,it's better than ending up suicidal because you don't use the only way your mind and body knows to cope..