What I have found is that most people don't know what to do in that situation. Even if they had been taught at some time, they have forgotten by now, or just don't think of it. Most people seem to think that they have to force their way into traffic, and the proper way to do that is to 'get their nose in' so the cars around you will have to give you room. Kind of like the automotive version of elbowing your way though a crowd.
Of course you were right; the only safe way to assist her to merge is to maintain speed and a safe follow distance from the car in front of you. Her job is then to merge with traffic at a safe place and speed. Any changes you make would only disrupt her ability to find that balance.
You think that is a funny situation, just go and watch a 4 way stop sign for awhile, eventually 4 cars will stop at about the same time, and NONE of them will know who is suppose to go first.
Or even better yet a 4 way blinking red light. Everyone seems to know they are supposed to stop, but then seems confused about what to do next. Some just sit there for awhile, often waiting to travel in the shadow of another vehicle, some stop and then immediately go, still others stop and then creep forward playing a little game of ?who gets to the center first.? Then there are those special people who don?t have time for all this and try to zoom around everyone else (into on coming traffic) or make right hand turns from the left hand lane.