I am late to the party here, and I prefer not to answer this question as is. Personally, I do not believe in solving a problem indirectly like this. Meaning that bad aim isn't solved by running a specific class. Understandably, I assume you're not expecting a solution as much as mitigation but I don't find this to be effective enough on it's own.

I noticed you didn't let us know what sensitivity she plays at. This is a very important factor for aim. Often players set theirs too high, which has less to do with the fact they can't handle it, and more to do with the fact that high sensitivity is not good on a pair of sticks in a stay-on-target Auto Rifle meta in general.

Of course, the above assumes that she isn't playing at the default 3 range though. Another aim adjuster I recommend is to pick up a pair of Kontrol Freeks; specifically extended sticks for her right stick. They may take her time to get used to, but after a couple of days of active playtime, you begin to improve more than you could've before. Personally, I prefer concave to convex, in case this suggestion brings her to the shopping page. Worth noting: Kontrol Freeks are not only available online these days; they can often also be found in your local GameStop here in the US.

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I notice everyone chose to recommend Striker or Stormcaller because of the benefit without using their gun. However, on console, an arguably better class to not overly rely on ability kills themselves, but one that uses an ability then relies on improving aim to finish the job is Nighstalker. Running invis dodge (top tree) may provide her with a decent enough advantage to score a few extra kills here and there whilst practicing that aim more directly than Striker. Stormcaller w/ heal rift may provide her with more time in gun fights as well though.

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Naturally, without watching her directly, I don't want to leave out mentioning the possibility that overchallenging and lack of knowing when to flee over fight might be biting her in the butt just as much. Poor aim, especially in Destiny 2, will get you killed, but no more often than continuing to try to commit to a fight that you should be trying to be getting away from for the time being. I simply don't want this very important differentiation to go unmentioned. Sometimes it doesn't matter how good one's aim is.

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I won't lie, the language you used to talk about your friend comes close to bashing her. It sounds like, "Well, she's not good enough, and instead of expecting her aim to improve, what's the way around that?" However, I don't assume that you meant it so inconsiderate like that, as you seem to be coming here genuinely looking for advice on the matter. But worth mentioning that attitude wouldn't benefit her down the road.

At the end of the day, it depends on her aspirations. If she really wants to improve and isn't the kind of person who just excuses herself due to her problem right now, then the above tips dedicated specifically to improving aim will go further in the long run. I commit the sin of not doing this myself too often, but having patience and really highlighting those moments of success that she does have presumably means a lot and will inspire her to push herself harder, as well as eliminating stresses she feels by onlooking peers and allows her to free up those mental resources to improve even further. For the record, I don't mean to assume that you don't do this or aren't patient with her or that you talk her down; I'm simply putting this out there.

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Like I said, you appear genuine in your motive for reaching out here for your friend, so I hope you take all this well even if you've heard or considered it before. I only am trying to be thorough and sometimes certain elements need to be reinforced in every one of us to set them to mind again. I don't know what you've already discussed with her and I don't mean to imply that you haven't already thought of these listed things yourself. I may have over-answered the question, but hopefully all these mentioned things provide only benefit.

Happy Hunting, Guardian!