In reply to mypyrex:
If she speaks to the guards on the platforms before getting on the trains, they will assist getting on and have a word with the guard about help getting off. Avoid the tube in rush hour. St Pancras services kent by high speed (approx costs a tenner more), is across the road from kings cross and half a mile from euston. Other station transfers are a bit more difficult. Avoid trains that are going to be inside the m25 before 0900 and between 1600 and 1830. Try to get to the london station at the start of the line and ten mins early as you will get a seat without the guards having to kick someone out of theirs.
Where are you going from and to?