Thanks for the reply.
So from what I can see they arrive into terminal 3 and need to get to terminal 1 to get the underground train(Db) S2 line to Stuttgart Hbf(main station) and then travel on to Heildelberg. I presume the airport has a help desk that could buy there tickets anyway. Don't want to buy them online in case the flight is delayed or they missed a connection. Once they get to Hbf it should be straight forward to get the on ward train via a help desk.