Liking multiple people at once is super common. The love triangle is a trope for a reason.
If you don’t like her then don’t worry about it (other than to maybe pay attention to how you’re acting around her and avoid flirting unintentionally) but if you’re interested in her, maybe try pursuing that? Flirt with her a bit and see if she reciprocates. If she likes you, there’s a good chance she’s been flirting with you and you’ve just been oblivious.
If you’re too shy to intentionally flirt, you could ask her outright, but it’d probably be better to ask her something that hints at your interest, like “I like this girl but I can’t tell if she’s into me - what sorts of signs should I be looking for?” Should be pretty obvious what you’re both saying and asking.
That’s worth considering, but it also needs to be weighed against possibly impacting their friendship if she’s not interested. I also wouldn’t recommend saying he “really likes her” if he doesn’t already like her a lot that way, but even just “I like you” would work just as well.