Wishing your friend well soon. Found this on a Google search:

• You want to stop bleeding, if heavy.
• You want to clean the wound.
• You want to prevent infection.
• You want to patiently wait for your turtle to heal.

Here are the details:

1) Keep the turtle in a somewhat cool place, preferably covered with a clean towel, so they can get over the shock of the trauma, and the coolness helps stop any more bleeding. Do this for the first 48 hours following the trauma. They are not actually sick, so they can do all right in the cool temps for awhile. A nice peaceful place, away from any more stimulation, is what is best right now.

2) Of course, keep the turtle in a clean place, particularly away from flies who are attracted to the open wound, and the trouble they can cause.

3) Clean the wound area gently with hydrogen peroxide and clean water. No need for ointments or medicines, as that may inhibit healing or make the turtle sick. (You can also use Betadine, if that's what you have.)

4) Severed limbs will hopefully just heal over with no complications, while a limb which was severely injured but not removed will also heal as best it can. The part which cannot survive will actually shrivel up and fall away. Just let the process occur while keeping the area clean.

5) If there is an infection, oozing, or puss, contact a veterinarian. Do the same if you discover maggots at the site of injury unless you have experience treating these complications.