My question is; In India can you buy a proper tank, filter, heater and test kits for those fish? Because that is what they need! Fin rot (shredded/disappearing/frayed fins) if that is what they have is usually a result of poor water quality & stress, it can often be nipped in the bud if water quality is improved and stress factors removed as much as possible. Does the white area look fuzzy at all? could also be columnarus.... here is a link to the diseases Freshwater and Saltwater Diseases
My advice, is that you stop listening to the LFS employee, do your own research and buy the appropriate setup/equipment for the fish as soon as possible or return the fish if you can't house them proberly, because honestly it is doing them a disservice keeping them in that bowl.
In the meantime, I would be doing at least minimum of 50% water changes every day to keep ammonia levels down, cutting back on the amount of food will also help. You really need test kits, if you don't have them the LFS may be able to test it for you. I don't know what the water is like in India, but if it has chloramines in it, you need a dechlorinator, also just letting the water sit overnight may not release all the chlorine, so the safest way is to use a dechlorinator.
Yes! The male can cause a lot of stress to the female if he is harassing her constantly. The ratio is 1m to 2-3 females, so the harassment is distributed.
Yes! You can feed those veggies (if feeding lettuce, normal lettuce has no nutritional value and isn't good for them, Romaine lettuce is best), blanched zucchine/cucumber is good and deshelled squished/cut up peas are very beneficial for fish and very good for constipation. Most veggies you should soften them in a bit of water in the microwave or 10-30 secs. first.