Wherever possible talk to the person concerned and support them in accessing help via the avenues listed in “Need help right now?”. If this is not possible, and you believe they need immediate assistance, it may be useful to call their GP, NHS 111 or in cases of immediate risk 999 (In the UK).
Caring for someone who is in distress can be a very difficult experience, remember to take care of yourself and perhaps consider getting some support for yourself from family and friends, a support group or your own therapy.