I’m sympathetic to OP/OP’s kid’s dilemma.
I think learning HS French/Spanish/German is of low value to many kids, especially if the kid knows some other 2nd language (i.e. Greek).
And, depending on the class structure of the MS/HS, taking a language can really bite into schedule flexibility. If kid is at a 7 class HS, and takes the big 4 (English, History, Math, Science) + Music + likely at least 1/2 time P/E, that’s 5.5 out of 7 classes, perhaps for all 4 years (except maybe the P/E). Add a foreign language, and there is virtually no space left in the schedule to explore other options. And I think exploring is useful, both for thinking about careers (Engineering, Health Sciences, etc.), and for life-learning (classes that are sometimes looked down on, but useful, like shop, foods/cooking, business/accounting, art, etc.).
OP has a better picture of kid’s interests and likely college goals than we do. But, if I were in OP’s shoes, I might be both reading admissions-page information on target college’s websites, and reaching out to admission officers (if possible), or HS guidance counselor, to find out how important an “official” HS language (Spanish or whatever is), and/or how the Greek fluency might be viewed. I think, depending on the specific college and likely path (STEM or humanities or whatever), OP’s kid might be ok without Spanish, but yes, perhaps some sort of “seal of approval” (not AP of course) for the Greek fluency might be useful.
If I understand correctly, many heritage/native Spanish and Mandarin speakers do study those languages and/or take the AP test in HS. It’s unfortunate for OP’s kid that modern Greek is much more obscure, not an AP, etc. But if the time is well spent, skipping Spanish for a broader range of HS classes might be both good for OP’s kid, and acceptable even to high end colleges. But I don’t know - I’m just a random CC poster - OP should research him/herself.