'thanks for your concerns and support. I’m in daily contact with her and I was already aware of most of your concerns (what’s going on and why she’s stressed) about her from her directly but I’ve told her that you’ve messaged saying you’re concerned about her and I’ve discussed everything with her and her husband but it’s not my place to comment further. Thanks.'
It's smug and here is why:
Insincere thanks
Boast - I am in touch with her 'daily'; you are not. I'm 'in', you are 'out'
I know what's going on from her directly so I am closer to her than you are
Told her and her husband that you messaged but they aren't interested ('not my place to comment further' insinuating they are not saying nice things about you)
If the friend had no agenda, the message would read more like this:
'Hi, sorry that Sally hasn't been in touch. Honestly she's okay, just needs a bit of time and space at the moment and I'm sure will be in touch when she's ready. I will speak to her gently when the time is right about how concerned her friends are but in the meantime please don't worry about her, honestly she is okay and her husband is taking good care of her.'
I think you are getting a very hard time here, I think it's obvious that this 'friend' is enjoying having your friend to herself.
Hopefully, your friend will come to realise that her old friends do genuinely care about her well-being. But honestly, pushing it anymore at the moment is only going to boost the mutual friend's position as 'gatekeeper' of your friend. You need to let it go and have faith in the friendship that you know is deep-rooted.
If anything, just put a card in the post to her to say that you miss her and then absolutely draw a line under it. Do not contact the mutual friend or the husband again. The mutual friend will use this against you.