Is repentance forgiven in the event of returning to the same sin, and what is incumbent upon one who abandoned prayer out of shyness regarding making it up, and how can one atone for lesbian acts and the remorse felt for them despite maintaining regular worship?

The lesbian acts you are engaging in are a grave sin and an abomination. Ibn Hajar considered them among the major sins. You both must perform sincere repentance (Tawbah Nasuh) and feel remorse for what you have done. We advise you to distance yourself from your friend to prevent corruption, and always be mindful of Allah's watchful eye. Repentance erases sin, and if a servant returns to sin, that does not invalidate their previous repentance. As for your friend abandoning prayer, that is a greater sin than lesbianism, and she must make up all the prayers she has missed.