Yeah it definately looks like TB- you should euthanise the fish as there isn't really any hope for it, its better you put it down now rather than leave it in the tank where it will eventually die and pass on the desease to the other fish.
Temperature shock works very well on guppys- simply get a bowl of water and put it in the freezer until a layer of ice forms across the surface of the water. Take the bowl out and break the ice and then net the guppy out and put her in the freezing water- the temperature shock will kill the guppy in seconds (usually takes about 4-5 seconds for me). Leave the fish in the water though for a minute to rule out any posibility of the fish regaining consciousness (its very rare though for this to happen, but its better to be safe than sorry).
After the deed is done, net the guppy out and wrap it in tissue paper and dispose of it as you wish (bury it, bin it etc- flushing it down the toilet is not advised); remember to wash your hands afterwards, people can get fish TB off fish- its not lethal for us like it is for fish, but will bring out a rash/sores on people and you may need to see a doctor- due to this make sure you don't have any cuts on you as well as you put your hands/arms into the tank.
There are various other humane forms of euthanasia if you prefer something else, you can do a search on the forum as there are many listed. I personally prefer the temperature shock form on small fish like guppys (i would not advise using it on fish 3inches long or larger) as it is very effective and quick on them and is not a gory or bloody form of euthasia.