My brother had zero bytes remaining on his C drive. How did that happen? is it a virus?

He was having issues with lag, apps wouldnt open, screenshots wouldnt work, etc. I opened his file manager and apparently C drive had 0 bytes remaining. it also said "NTFS" below the storage bar which scared him (i told him its a file system, he didnt gaf).

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I tried to delete some images to free up a little bit of room, but it literally wouldnt delete. I ended up installing wiztree on a usb to double check, and yep, zero bytes. I used wiztree to delete some files and all is well now, but my brother insists he did not do this.

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he has trend micro installed, which we ran after clearing some space and it found nothing, and he insists he is safe on the internet (he is not, but i seriously doubt that this is a virus based on the files i saw in wiztree. his computer is literally 40gb of windows and 450gb of steam games)

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his current theory is that the empty, freshly reformatted usb drive i lent to him 6 months ago had a virus on it which filled up his storage because "windows shouldnt be 40gb, thats a virus". this is not true, but he insists. please back me up here. help