Your voice and your friendliness are what they sought out. DO NOT call the emergency services on them, especially not cops, without their express consent or unless they took or did something. Cops and medical services are usually putting trans people more at risk than anything.

Best advice I can give is for starters, make sure they didn't take or do anything yet, then make some coffee and talk them through until they're sleepy, then you can check on them and have other friends check on them later. Don't lie to them, just talk, there is no reasoning in a state of distress

Anyway, make sure your trans friends have a few people to call in case of emergency, you CAN'T and SHOULDN'T rely on or have them rely on ONE person, it's unsustainable.
Suicide hotlines aren't trans friendly at the best of times, but make sure you have their number somewhere just in case.