Dog problems don't read and not reply if you have possible answers please!

Does anyone know how to treat a dog for gingivitis without going to the vet? It isn't my dog but I'm watching it while they deal with a family emergency in a different state. I have this dog until march. I contacted the owner and she said she can't worry about it right now bc she already has one vet bill to worry about from the same dog last week. The owners husband took the dog home a few days before bringing it back and she got into something in that time frame and got severely sick. She isn't going to make the effort to help her dog and I'm left with it. I don't wanna shell out my money for a dog that isn't mine without exploring home remedies first but I won't let it suffer either so if it comes down to it I'll take the dog in but does anyone know of anything I can do to treat this myself?