I have guineas and they are extremely difficult to do anything with. I would probably try the internal route. I would do comfrey internally and if you have it growing you could just chop it up and feed it in their normal ration. Or you could make sure they get it by making oatmeal and putting it chopped fine in that and they will gobble it up and never know they ate it. LOL! Most likely the bird got into a scuffle and pulled a muscle or tendon. They are notorious for picking on each other, and as a side note, if you try to catch them (stressing on the word try!) make sure you don’t hold them by their legs because they will injure themselves by thrashing violently around. I had to save my female from a net she got caught up in and she was thoroughly incensed that I would dare to try to pick her up! So between trying to hold her around the chest without getting mauled by her sharp claws I managed to get her free, but not without some gashes to my arms from her.  Personally, I would not try to take the pain away as long as it was not too bad because it may prevent them from injuring it further, but you could add cayenne pepper, tummeric, ginger to the mix would help with inflammation and pain.