So sorry to see this injury.
I am a pre-nursing student and what the nurse probably told you was that the Synovial fluid inside the joint capsul has begun to leak from the joint.
In order for this to happen you would more then likely if the horse tolerated it be able to see inside the wound down to the joint.
Synovial fluid is a viscous(sp) type fluid similar to the clear stuff that comes out of Chiken when you bake it...sorry maybe not a good analogy.
I think 7ponies has a great suggestion of cleansing this wound with saline soulition for while and maybe you would try to use a gel ice pak on the knee.
I totally understand that you do not have the money but I would want a vet to come and look anyway. I am sure there will be a way to get it worked out financially. Either payments or some other means. I know how you feel as I have in the past been in that situation.
But- it is important to treat this injury properly. I would think it would be best.
As far as tolerance to the antibiotics....well I highly doubt that one round of antibiotics is going to create any tolerance issue.
Maybe you can call and discuss this injury with the vet and see if you can e-mail them the photo's. Then they can possibly tell you what to try doing and keep the cost down for you. Some vets are very cool about that stuff.
Making sure you horse gets adequate care will be best and if the vet can help vis e-mail or phone you will have better direction to take care of the injury!
It could take some time for this injury to heal especially if the joint capsule is broken and the synovial fluids are not present. The vet should be able to at the least talk with you and if one will not Try another one...okay!
Best of luck to you and hang in there...
Oh- do you have a syringe without the needle? A big one??? If not you may be able to get one from the vets office or a local feed store.
If you can find one take the saline solution and load the syringe. Then flush this would out! Do that once or twice a day for a few days.
Main thing is to get those pictures to a vet to look at. At least see if they will.