It's very treatable, as people have said.
Some advice since someone very close to me (who I won't mention specifically as this person is entitled to a little medical privacy) has been dealing with Hodgkin's Lymphoma. Similar though not the same, but some things apply.
Your cousin probably won't just be quarantined this one time, as having a cold or the like can be a lot harder on her than on a healthier person. Plus chemo can be put off if the patient has a cold. Yes, even the common cold. So your cousin may go out wearing a medical mask, or not be able to hug or even shake hands. It's nothing personal.
Nausea sucks, of course, but sometimes it helps to smell something nice but not too strong. Flowery aromas might be too much but the smell of cinnamon might be okay. Small doses are best. My patient
liked potpourri. Your mileage will vary in this area. Warn if you're going to send or give such things before you do. Sometimes things that are okay for a while stop being okay later. And then they're okay again.
Foods aren't appetizing but that doesn't mean that nothing is. Italian worked better than Mexican or other types of food. Again, mileage varies. Strong spice of any sort is probably best avoided.
Fatigue is common. Don't overdo or let her overdo, even if she insists. Try not to treat her like an invalid but basically don't get her into situations where there can be a lot of fatigue. E. g. if you go walking on the beach, say, go about 1/2 to 2/3 as far as she thinks she can walk. You can always walk more later. Attitude is important, and she'll feel a lot better if she goes a shorter distance and has success, versus if she attempts to go a longer distance and doesn't have success. Try to be cued in to fatigue as it can come on very suddenly.
Hair loss may or may not happen. My patient
liked baseball caps and bandannas, but also had a wig. Perhaps an assortment of colored bandannas would be welcome -- of course, gauge what you think would be welcome. I would sometimes wear a bandanna (like a do-rag) when I saw this person.
Talk about whatever. Chemo and cancer are big parts of her life but they aren't the only things going on in the world, and she might sometimes be a little sick of talking about it all the time. The Red Sox, the weather, the election, your job, your family, etc., there are lots of things to talk about, rather than the elephant in the room.
Good luck to her and to you -- it can be hard to be the support person, too. Send me a PM if you want more info. I've been through this as a support person.
PS Chemo is cycled, then she'll get a CAT scan or PET scan or just a checkup to see how things are going, to determine whether to continue with chemo, ramp it up, ease off on it or change the drugs or add radiation. For my patient
the cycle's 4 rounds of chemo. Your mileage will vary.