Part A Only and ACA Subsidy

I do exactly zero ACA biz and had this situation come up recently. Person on SSDI qualifies for Medicare but wants to keep her $25 (after subsidy) ACA plan. Tells me that she was told that if she opts out of Part B she would be able to do this.

Now I'm 100% positive that a Medicare beneficiary with both A and B canNOT also have a subsidized ACA plan (why would you?) but with Part A only is it doable? I've been Googling but results are conflicting.
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