Some extra pictures
What's the watering like? Does it keep happening to other leaves?
I believe she waters 1/2 cup every five days? She uses an app that tells her when to water things. It doesn’t appear to be spreading to other leafs, but it might be growing slowly onto the leafs that are already bothered. I just got it a week ago and haven’t noticed to rapid of a spread among the already infected leafs.
That could still be too much or too little depending on soil and air conditions. A quick Google says they like slightly moist soil. If it's soggy, that's too much water. If they dry out, that's too dry. (Too dry is easier to recover from, but from appearances, it's probably too much rather than too little water.)
If it had plenty of other growth, I'd say you could remove those leaves, but it's not really big enough for that.
I don't think it's actually an infection. The only other possibility would be unsuitable soil conditions. I assume at some point it was repotted with fresh fertilized potting mix, and wasn't over-fertilized, though that could also cause what you're seeing.
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