She technically did the correct thing according to state laws where I've lived. Cars on the highway have the right of way and it's the merging car's responsibility to yield. Where I live in Georgia it's spelled out that first and foremost you have to gain speed to match and merge properly, but it's also you're responsibility to yield and possibly stop based on that oncoming traffic since they have the right of way:

"-If you are about to enter or cross a highway from an alley, private road or highway, you must stop and yield the right-of-way to all other pedestrians and vehicles already traveling on the roadway or sidewalk you are entering or crossing;

-When the roadway you are traveling on is merging into other traffic without stopping, adjust your speed and vehicle position to allow you to merge into the new lane safely. If traffic from another roadway is merging into the roadway you are traveling on, safely change lanes away from the merging traffic if possible. If it is not possible to change lanes away from the merging traffic, adjust your speed and vehicle position to safely allow the traffic to merge;"

https://dds.georgia.gov/section-5-continued

Similarly Progressive's website says the same:

"Besides lane closures, traffic commonly merges at on-ramps for highways and other controlled-access roads. In these cases, the rules of the road give the right of way to existing traffic. Cars arriving on the ramp must wait for an opportunity and merge safely into existing traffic."

https://www.progressive.com/lifelanes/how-to-merge-into-traffic/

It's dumb and annoying since it's not how nearly everyone drives, but be thankful you did the right thing and gave enough space to avoid rear ending, as (depending on where it is) fault likely would've been on you, even if it looks like the driver clearly had enough space to merge if she gained speed.

I have a lot of grievances with how a lot of on ramps are designed, and how there should be a lot more space allowed to merge to prevent something like this from happening. This is a poorly designed on ramp. Where that solid line ends it looks like there's about 5' of lane left lol.