It takes all the time pressure off. He/she won’t have to worry about getting to Katahdin before it closes, or dealing with cold weather in New England or summer heat in the south. It’s much nicer to be able to hike without worrying about a schedule, or having to make big miles whether you want to or not.
AS to the original question - it really depends. I used a 20 degree bag for the entire trail. I was cold a couple of nights in the Smokies and in Maine, but I was usually fine. In summer I just slept on top of the bag or with it wide open. It was about 90 degrees when I picked up the warmer clothes I’d sent ahead to Glencliff and it was really hot again when I left Gorham but I was glad I had them when I had to deal with all day rain going over Mt. Washington and a couple of cold wet days in Maine, but a lot of the time they were excess baggage. But that was in September. You might be able to get by with a 40 degree bag until September, if you have a warm jacket, hat and gloves with you. I’d certainly pick that up in Hanover, if you aren’t carrying them with you all along.
Ginny