Tell her loved it and ask who filmed it for her. Don't jump to any conclusions and don't approach her in an accusatory way. It could very well have been a female friend.

Two birds with one stone. She's thrifty at least

Look at when the file was created. Could be an old one

Mabey she's holding it.

You know what they say: Never look a gift horse in the ******.

Check with her before you get uptight about it. Unless you're talking blatant tells like zooms it is possible her homemade "tripod" was just wiggly. I have made Dr-Seuss-looking stacks of assorted objects to get a camera at the right angle & height for a selfie. As someone else has said, even a dresser or nightstand can wiggle if it's close enough to her. Was the angle such that the cel could be next to her on the bed, elevated on a pile of cushions? Or she may have done something really creative like tie her cel to a tall houseplant, or dangle it on a string from a ceiling hook. People do all sorts of goofy stuff to get a selfie. It's more likely that she did it herself in a weird way, than that she's cruel/stupid enough to send OP proof she's cheating, or than that she masturbated in front of a platonic friend.

I think that OP needs to follow up

Maybe it was one of her girlfriends?

I think there is too much debate on how it was taken when nobody has actualy seen it. I think it would be best if you upload it to be sure.

I'd probably compliment her in the angle and ask how she managed to film it. Gets an answer without an accusation.