When I was a child I had real issues with this. To this day I can't do mental maths. I also was fairly good at times tables. At primary school they held me back in maths because of it despise being very good as I was near the bottom of the class in mental maths. My parents tried to help me at home but unfortunately I didn't find anything that helped and I remember getting upset about it. It was extremely frustrating and if anything the pressure of parents trying to help me just frustrated me more because I couldn't get my head round it.
I went on to be one of the very best at maths in my year at GSCE and in the past my work has involved a lot of bookkeeping. I just have to write everything down to be able to 'see' the problem. I can't visualise it in my head, if that makes sense. Its very difficult to explain.
Essentially my problem was/is that I can't recall numbers. I also have trouble reading them out loud or writing them done. I frequently get digits the wrong way round when I speak out loud. For example, some one will say 5,7 and I'll write down 7,5. So telephone numbers I find a nightmare. I have learned to cope with it by also repeating numbers back to someone when I write them down to check I have got it right; something I found very stressful when I started work but have learned to cope with.
I suspect in my case it is probably some sort of learning difficulty, but its offset by the fact that I have also have a high IQ and ability to understand the principles of maths so I learned to cope with my weakness early and it was never picked up as being a problem for me beyond primary age. I believe its possibly some sort of mild dyscalculia, but obviously I have never been assessed and it didn't cause me issues in the long run so I never really thought much about it. I don't display any other common symptoms of dyscalculia either; I just know there is definitely something 'not quite right' with how I think about numbers compared with the average person. Certainly I know that DH was the same with English. He was diagnosed with severe dyslexia as an adult, but achieved highly because he had a high IQ and developed coping skills and strategies. He struggles with reading, but you would never ever know. So I know in theory, the possibility of me having a learning difficulty is there and isn't restricted to people who struggle with words or numbers.
With her being good at maths and times tables, but not being good at mental maths, I do wonder if she has similar problems to those I had. I think you might be wise to be cautious about how you approach trying to help her though, with that in mind. I would definitely try and identify what aspects of mental maths she is struggling with first. I tried and know all the tricks above - (they are definitely worth learning as they are helpful for maths in general) - but knowing them doesn't really help with my mental maths, because I can't visualise and recall numbers. They just get jumbled up in my head.
Sorry, this isn't more helpful, but it does sound similar to my issues with mental maths, so I thought it was worth at least putting forward the idea of the possibility. Of course, it may not be the case and you may find a way of helping her.
The only reassurance I can offer you, is that not being good at mental maths, isn't necessarily going to hold her back in life. Provided she is doing well at maths elsewhere, it wouldn't concern me hugely.