I accidentally did this once with my recent ex and my aunt. I sent it to her instead of him. It was really simple, "I will, Baby. **Snuggles**", but it wasn't exactly something I'd say to my aunt, iether.
She responded with a, "Will what?" and I turned about 10 shades of red. It was a pretty big joke at Christmas.
On the flipside, though, if she doesn't realize that you got it instead of him, she may get upset at the lack of answer, and if it's an ongoing conversation, the flipside may happen too. (Depending on their ages and the length of their relationship and everything.)
Maybe just let her know to resend that text to him, somehow?