How to handle an overpowered PC
Ok, context: I'm a first time DM running my very first campaign in about a week. Everything is going smoothly so far with planning and scheduling, but I've run into one major issue. My girlfriend, through sheer dumb luck, has created the strongest 1st-level character I've ever seen. She's playing a Kalashtar cleric, and her stats are as follows:
STR: 16
DEX: 15
CON: 14
INT: 18
WIS: 20
CHA: 16
Along with these stats she has an AC of 16, the features Dual Mind, Mental Discipline, Mind Link, and Severed from Dreams. She also doesn't require components as her holy symbol can be used as a focus, and she has a spell save dc of 15 and a spell attack bonus of +7. Maybe I'm overreacting because of my own lack of experience, but I'm worried about a few things. How do I give her realistic challenges while also keeping the campaign fair for my other (admittedly weaker) PCs? I could always go the route of free equipment and bonuses for the other players, but this feels like bullying her for accidentally making a powerful character because she "went based off what looked and sounded coolest" for her first ever character. Sorry if this post is too long or badly formatted, it's my first ever reddit post and I'm on mobile.
TLDR: One of my PCs accidentally created an overly powerful character and I'm not sure how to keep my campaign fair, challenging, and fun.