This is a tough one for us to help you with given you are not even able to replicate it yourself on your site. Let us know if you are able to get replication steps, or see if the user who is having the problem on your site can find time to upload a few images to this topic here on meta. The outcome of that is something we might be able to work with.
I have an iOS device myself and have no problem uploading multiple files. Maybe the issue is with bandwidth and/or file size? I’ve had problems in the past uploading images when on a slow connection.
I’m going to set this topic to close automatically in 30 days, given the problem seems to be limited to this one person.