After I login to a server, I continuously fall through the ground.

Solution 1:

This is due to connectivity issues. You are "lagging out" so the server isn't receiving any response from your client and you aren't receiving any information from the server meaning nothing is happening.

Other players will see you just standing there, and you will see "nothing happening" despite everything that everyone else can see. If, like you have said, You are having internet issues try contacting your ISP (internet service provider, the company that provides you internet connection) and they should tell you if everything is ok on their end. If they say everything is ok, you may have to replace your router (I recommend buying a decent one as an investment. Cheap ones tend to work for about 6 months and start declining in performance based on experience.).

As stated by GOD_is_AFK__, you can restart your router and this will tend to temporarily solve the problem. Also ensure that you don't have above 5 devices connected to the internet as it may slow down everything due to traffic.

You can also try other servers to see if the same thing is happening. If it isn't, it is your friends connection. If other people play the server, ask them what their connection is like (are they lagging?). If they are aswell, your friend may have to do what I have just stated above.

Maybe try and run your own server and get them to connect to it and see if she then has the problems.

Solution 2:

Try to restart your router (if you have one), that worked for me sometimes.