Yes it can cause problems. A rider slamming on her brakes in front of me, downhill, on a sportive, was the cause of my concussion and broken collar bone. This has resulted in my own nervousness downhill although I'll still do it at speed, but I am very wary riding with other people now.
It will take some time for her to get used to it but I would avoid encouraging her to ride in groups until she's more confident.