I've never come across it Mel, but I would think that if she's signing for her angel, then it wouldn't be upsetting for someone else to acknowledge her as well - if it were me I would find it incredibly sweet, and give that person a rib squishing hug and a thank you next time I saw them!

Perhaps she's still signing because she doesn't want OTHERS to forget - and therefore by you acknowledging her angel, she might feel more comfortable to let her go, as she knows she won't be forgotten.