Welcome to the forum....
It is important to know why the Betta got fin rot and fix that-otherwise it could be an on-going problem.
If you will fill-out this info sheet and add it to your thread-we can help you figure out the cause-unless you already know....But we will need more info regardless.
It sounds like you already have a good idea and instincts on what to do.
Good that you are using a QT for treatment-IMO/E-in most cases it is almost always a good idea to QT for any treatment, however, there are some exception.
Yes, it is a good idea to add the silk or live plant to the QT to help make him feel more secure.
Can you post a pic and/or describe what you are seeing. Depending on the degree of damage/infection will determine the treatment-If it isn't too involved-often a clean water and tannin treatment is all that is needed-along with good nutrition. In severe cases-I like to use aquarium salt 1-3tsp/gal, tannins and good nutrition.
Depending on his regular aquarium with the ghost shrimp-several factors regarding the need for 100% water changes-Once we get more info on that aquarium we can advise on those needs.