Is this a "Do you like Claire?" litmus test we're looking at right now?
Nah, I like Claire, mostly. If anything, I am arguing that Marten did wrong here.
First of, I don't care about the online banking stuff for the reasons mentioned. I am on about messages in particular.
I find letting people look through your messages/have access to your phone an icky thing for two reasons in this situation:
a) It is unclear to me if Marten allowed Claire to access his phone or if he just doesn't have a pass code set and she is using it willy-nilly (in that regard, my original reply could have been better worded). If that is so, that's really not kosher of Claire, and negligent of Marten.
b) If he agreed to it without reading the message, I consider that betraying the trust of whoever sent it. It is somewhat different if he read it first and then went "ugh, my head, you go reply to that", although there is still no reason why Claire would have to use his phone for that - I assume they all have each other's phone numbers by now.
If I was in the sender's shoes and received such a text from someone else than the intended recipient, the least it would trigger would be a discussion along the lines of "does your partner read all the messages I send to you?" Making people wonder that is really not kind, although it can be forgiven (particularly if Marten is not thinking straight due to hangover).
Needless to say, I don't even give my partners access to my phone, and I feel icky when I have to use my girlfriend's phone, because sometimes notifications and partial messages pop up while I am using it, and I would really rather not read those.