My friend has flown US domestically alone and was OK. The main concern here is 1) making the connection between flights (no way to avoid at least one) and 2) the less friendly design of many European airports (lots of stairs, especially at arrival gates). My friend can likely manage getting on and off the plane solo, and no problem with lavatory, etc. Otherwise, my friend is quite independent, drives alone, shops, cooks, cleans, etc.