Best answer by Airgetlam
To the OP, if you're playing music "from this iPhone", Sonos is tracking a bug about that, which I would expect to be fixed in the next release of software. They've recommended a couple of potential fixes in several threads, the one that sticks in my mind is that you turn off the autolock function for your screen. I think there's another one, but I don't recall exactly what is is, and there are a myriad of threads in which they've explained it.
That being said, both of you might benefit from submitting a system diagnostic within 10 minutes of replicating this issue, and contact Sonos to discuss it. There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you both find a solution, which might be different for each of you.