Avoiding the Whitewashing of Non-appearing POC Characters
I don’t believe it’d be completely odd to note/describe their race in a straight-forward manner, that is if you did this with the majority of the characters you described. I’ve read countless works where characters familiar to the narrator are described this way; if you think about it in terms of the narrator introducing the character to her audience as opposed to the MC observing these details as if they’re new, it’s not so strange.
If you don’t prefer that route, though, another way you might go about describing them is if your MC recalls a memory or flashback of her friend(s).
If she’s “there” in the flashback, she might note their appearance, and if integrated into the action,even better. Say she notes Kim pushing her coily hair behind her ear in frustration, or the sheen of sweat against her dark brown skin.
If your MC has ever visited their homes and there’s any cultural décor to note, you could use that too, but I wouldn’t put it there just to be able to reveal their race but only if it’s actually integral to them or their family’s lifestyle.
You mention she calls and texts them often; but what about video chats? If she’s seeing their faces appear on a screen, there’s another natural space to describe them.
Another chance to describe them is if she has photos of them around or sees their pics on a social media site.
Hope that give you some ideas! Browse the description tag for more posts on indicating the race of characters.