I spend a lot of time with my friend and her teenage children and we're close. We regard them more as family than our actual relatives.
Both of her DC are wonderful and super polite and sociable. I often take her 15 yr old DD for lunch/coffee just the two of us while her mum works and back in October she mentioned in passing a product she really wanted but that you couldn't get in this country. I contacted a friend who lives in the US that night and arranged for her to grab it for me and have it posted over for her Christmas.
Just before Christmas I dropped the gifts round to their house and DS (12) was home and gave me a cuddle and thanked me and put them under the tree. Their mum gave gifts for me, my OH and DD. All very thoughtful and lovely.
On Christmas day her DS sent me an excited Snapchat (he insists I have it on my phone) to show that he loved his gift. Their mum Facetimed us on Christmas Day and we thanked each other and my DD was very appreciative of her gift and told my friend that.
I've not heard anything from her DD and I find that really strange. She's active on social media and frequently posts comments on my photos and she messages me often. But she's never mentioned her gift.
In the end, I got it for her because I knew she wanted it and I wanted her to have it. But would you mention to mum that she didn't acknowledge it? I would be mortified to know my DD didn't say thank you for a present.