There are some simple experiments that you can do to demonstrate the shape of the Earth. You can watch things disappear over the horizon as they move away from you - ships being the standard example. You can drive toward a tall building or a mountain across a nice level plain and watch as it comes into view top-first.
If you're in the West you can go to the beach and lay down near the water, watch the sun set, then stand up and see it set again. In the East you can do something similar, but watch the sun rise while standing then lay down and watch it appear again. The differences in both cases are quite small so as soon as you see the first sliver appear in the East or the last bit disappear in the West then change your viewing height.
If you live anywhere near New Orleans you can go to Lake Ponchartrain and see the curvature of the causeway or go a bit West on I-10 and check out the power transmission lines. 204
Take some binoculars or a small telescope to get the best view of the curvature in the distance.
If you're close to the Equator then you can spend some time in the evening watching the Northern and Southern stars circling their respective celestial poles. Or you can watch the satellites transiting the moon - sites like TransitFinder can help with this.
I could go on and on. There are many many ways you can test this.
But before you try any of them I strongly recommend that you first take the time to find out how deeply into Flat Earth your friend is. If she is watching all the normal Flat Earth videos on YouTube then she almost certainly already has trite answers for every observable phenomenon that you could possibly show her. If she's a True Convert to this pernicious little meme then there is likely nothing you can do to change her mind.
The harsh reality is that there are many people out there who will believe in the face of all evidence to the contrary. Nothing you can do will change them. 204