YIKES!
Sadly, this is NOT a wound that will heal on its own so you’ll have to make a decision to act.
A vet is your best option if possible.
It’s understandable
Thank goodness your friend is coming with an extra set of hands today.
Speaking of hands. Wear gloves!
Opossum’s mouths are filthy, but the good news is it is extremely rare for one to carry the rabies virus.
As stated above by other posters, this wound must be cleared of feathers, cleaned and evaluated in bright indoor lighting.
If the skin wound is big enough, you may be able to sort of hold the wound open with a pair of clean tweezers or forceps if you have them, and peer inside with a small flashlight for a hint of the trouble inside. If it’s not big enough on the outside, and you’re going in for a repair, scissors (that have been cleaned or sterilized) are the safest and most humane way to create space without potentially “slipping” and having a heart-stopping “oh crap” moment with a scalpel. Think of your exterior opening as a window or door.
If it is an esophageal tear and its small and up at the higher end it’s possible that you may be able to glue it shut. practice pinching it together. Can you?
Does it come together neatly or is the tissue ragged and torn?
If it’s esophageal and farther down, it might be tricky to say the least.
If food is leaking to the outside I’d be willing to bet it’s the crop that’s torn which honestly,
I would hope to discover!
A crop repair isn’t a catastrophe!
The cavity inside her body in the vicinity of the source of the leak and wound would have to be flushed very well with a diluted betadine (povidine/solution to kill germs and hopefully ward off infection.
Edited to add: somehow I lost my train of thought early on but I wanted to say ...
It’s understandable if seeking the treatment by a vet is not an option. Byc’ers understand that and that’s why we’re here to help guide if we can.