What do you think would have happened if it had been Garp and his marines to find Kei instead? Just wondering.
In all likelihood, a fight. At least here and now, if we swapped out the Kuja Pirates.
Even if Garp didn’t tend to greet his grandsons with the Fist of Love straight to the noggin, the Marines as a faction are (at least nominally) trying to kill absolutely everyone traveling with the main characters right now. There’s a lot of prison escapees, infamous pirates, and Revolutionaries in that group. All of them are kinda supposed to be “shoot on sight” for the Marines.
Kei and Isobu only know Garp as “Ace and Luffy’s grandpa who still left Ace in prison” and have both been primed to attack Marine vessels due to their experiences. And due to Saiken and Utakata’s experiences. And they’ll be backed by Matatabi and Yugito the whole way. And you can bet Garp the Fist/Hero wouldn’t take an attack on his men lying down. He’s one of the decent Marines we see in the story, and while there is little chance he’d actually hurt his grandsons meaningfully (or get in Luffy’s way much during the canon Marineford arc) most people on both sides don’t know that.
But if you’re referring to back at the beginning of the story, if Garp replaces the Whitebeard Pirates? That is another question entirely.
Kei would probably hang around at least for a little while, hiding Isobu to keep him from scaring people per usual. While it’s been a very long time since she’s met a faction calling themselves “the Marines,” she does have a moderate lingering respect for sailors. This respect would last approximately until she reached a place where the corruption in the organization bubbles to the surface. (Less of an issue for pirates, because no one expects them to be saints.)
At that point, she’d ditch them. Especially if they at all interfere with her mission of finding the Tailed Beasts and their partners. Given how almost half of the jinchuriki end up joining pirate crews (Utakata, Gaara, Kushina, and B) sticking with the Marines would be actively detrimental to her cause.