Rough idle

A friend put in a new battery today after my battery died. It starts fine but when I approach a stop light or put on the brake it idles very roughly like it is going to shut off. Then when I accelerate, it is fine. So he told me to let it run for five minutes or so and it seemed to be fine. After work it starts right up but after about five minutes it starts the rough idle again when I slow for traffic even after I drove it more than eleven miles home, with each slow down and braking the idle seems like it is going to shut off. How can an older woman like myself reset this without being ripped off by a shop? Help! Thank you!