This is the first speculation that I have heard of this reason for her leaving the arena, and it seems plausible to me, especially in light of accounts from another recent concert there that text messages were delayed for up to 2 hours, iirc. If she simply went to the bathroom, then was wanting to text someone or was expecting a text from someone, she might have stepped all the way outside at an unguarded door (are there any at a concert at JPJ?) to get reception (not knowing, forgetting, or not believing the no reentry policy), or stepped halfway out, as you surmise, with the door slamming shut (or in some other way ending up locked out unintentionally...a guard seeing her halfway out and saying "half out is all out," or some such, though LE says there is no evidence she was asked to leave).
Apparently, though, it was an actual phone call, not texting, at 8:48, placed by the friends, so presumably they had cell phone coverage inside and she was not at that time being held against her will.