Torn toe-nails are usually not life threatening unless there is uncontrolled bleeding. I would not cut the nail myself but let the vet take a look at it and see if it needs to be removed surgically.
If the area is bleeding uncontrollably you will need to get him to a vet or animal hospital right away. If there is only minor bleeding you can apply direct pressure firmly until it stops. You can also use plain old household flour to help control any bleeding.
It is in the cats best interest to be seen by a vet in the morning if possible. You are already doing the right thing by placing him in an area where he can not obtain further injury but be sure to check on him frequently. If there is an area of the nail dangling that is not in the area of the blood then you can safely trim that area. There is a vein in the lower part of the nail that can cause constant bleeding. If the wound is in this area it is best to let it be and let your vet do the exam and removal if necessary.
If you have any further questions at all please don't hesitate to ask.
Sincerely,
Bunny -- Veterinary Technician
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Veterinary Assistant
10+ years at a veterinary clinic as a veterinary assistant. 20+ years raising/showing rabbits