I was a headstrong, unhappy but academically successful 'sensible and mature' 15 year old when I got with my 21 year old boyfriend.
He was a selfish, insecure manchild who love bombed me to give me the sense of belonging I craved, and emotionally manipulated me for 3 1/2 years. I was 'legal' for most of the time I was with him, but that didn't make me less vulnerable. My parents didn't like the sex part when I was underage, but were too scared to try and stop me from seeing him, so he stayed in my home when he visited. They knew we had sex, so they put me on the pill, but they didn't know he would show me porn to teach me what he liked, and fetishise me in my school uniform. They didn't know know he would put his hands around my throat and throw things at me when we spend time at his place, or that he made false reports to the police about me if I left.
Over the years, when I would sob because we would row about me having my own life (a job, college work and a few friends), and he would so often threaten to leave me because my independence hurt his feelings, my parents didn't know what to do for fear of pushing me away, so they comforted me and said relationships are hard and take work.
At nearly 40 I'm still dealing with the emotional control and training this relationship impressed on me at such a formative time. I wish somebody had had a really frank conversation with me about what love is and is not, and what control can look like from someone who plays the victim, uses your mental health against you, and tries to isolate you.
Not all age gaps are a power imbalance, but there's a distinct possibility they can be, and men who start relationships with children are often doing so for specific reasons which they can be good at hiding/denying. I wouldn't want to take the chance of letting the same thing happen to another young person that happened to me.
I'd say yes, do make it clear you're there for her, but also talk about some examples of healthy and unhealthy relationship behaviours/dynamics, and try to keep this conversation open and ongoing. Use cultural reference points to bring this up if need be, and say clearly how you would hope those scenarios would be handled.
Even if she doesn't tell you the details of her relationship, she may recognise things you describe and it may equip her to question anything untoward early on, maybe talk to someone about it, and even establish her own boundaries.
Nobody really told me I could have boundaries and still love someone. Or that I could have hard outs, and this wouldn't make me cruel or selfish person, until I was completely wrapped up in this dynamic and couldnt see a way out.
Young women and girls need to hear this stuff, repeatedly and clearly, especially if they're in relationships with men. Even if they are already 'sensible and mature' for their age.