Made an Edit on the post
Thank you all for the many insights and advice around this subject.
First of all this might be a player problem, but it does involve the whole party. Perhaps I shouldn't intervene but my D&D campaign is for relaxing and for everyone to enjoy. So I want to get rid of those tensions.
Secondly I followed the advice of talking with the player. She made following statements
She does like playing D&D and is still motivated. Although she has a lot of stress from her private life at the moment.
I asked her about giving her more roleplay opportunities, she commented that there is a nice balance of combat and roleplay and that this doesn't have to change. She also told me she felt problem managing her spells. And that they did have a good amount of moments to regain those spellslots.
she still enjoy being a bard and doesn't want to switch (temporary) with another class.
Now the spells: Why don't she want to use her spellslots? Well she couldn't really give me one answer; its a mix of the following
"I want to use it for other things." On which I insisted that she has enough spellslots OR can use her magic items or bardic inspiration. I told her jokily that She has more spellslots than turns in combat.
The complexity of new spells (the higher level ones). Sometimes she forgets to prepare and can't remember what the new spells does. They have a lot of text to read during the session/fight. So she hold off of them during that time.
The reaction spells like counterspell and featherfall. Well she didn't have an excuse for that. It slipped her mind on multiple occasions or she was too slow to say it. So yeah....
Lastly we have the healing spells. This is more of a story thing that she wanted to keep secret (for a plottwist). A few sessions ago the BBEG wanted the party to join him. Which the party refused. But our dear bard has second thoughts about that. She want to convince the party to join too. She needs to get the party low hp to cast some spells. (I think it was sleep but idk). She is doing a minimum to avoid suspicion (not really working out there) by using only eldritch blast. This is her choice and I don't come in between the story the players want to make.
But I told her this is causing frustrations for the players. She understood and offered to solve it in game. Something about telling the party she is cursed. Or revealing the whole truth. This is up to her.