OP YANBU to report the skiver, but YABU to be so proud of being a telltale over any issue.
In this instance, I do think that you should have informed the manager, it's one thing going home a bit early, or call in sick (at least that way they can try to arrange emergency cover), but going home after 30 minutes and then texting you about it is just rude when you're having to cover for her (along with everyone else). But again, it depends how you did it, if you immediately went to your manager to be like "Manager, Skiver is skiving, she just sent me this message!" then it just sounds like you're wanting to get them into trouble. On the other hand,, something like "Look, I know it's not my business but I had to stay late to cover Skiver's shift, which I didn't mind doing if she was sick, but she's just sent me this message which makes it clear she isn't and I'm annoyed and just wanted to let you know that, and that I won't be covering for Skiver ever again"
Having said all that, I also work in retail and every shop floor colleague, at some point or other, has cut corners on a job or taken an extra long break or skived in the back room for five minutes ... if there was a colleague who "didn't hesitate" to report us for that kind of thing, then they wouldn't really be trusted very much.
It's one thing if your colleague has gone on a 15 minute break, your manager comes over 30 minutes later and ask where they are and you say "Don't know, they went for a 15 min break about 30 mins ago and they haven't come back yet" because you're just being honest, but another thing to go running to the manager after 17 minutes because they took more time than they should have.
I will occasionally "exaggerate" the time something is going to take, if I need time off, because I know that work will give out as little time as possible and often won't be punctual with cover so you can go on time. If one of my friends grassed me up then I'd be pissed off, I only do it because I know how unreliable the system is not to get any real advantage ... eg I had an important hospital appt at 4:45 a few weeks ago and it takes 10 minutes to get to the hospital, but I wasn't due to finish until 5pm. I spoke to my line manager a few days in advance but told them that the appointment was at 4:15 so could I leave at 4pm? My manager said that was fine so I reminded the supervisor at the start of my shift that I had to leave at 4 because I had an appointment at 4:15, no problem ... 4pm comes and goes and nobody came to cover my job so I had to call the supevisor who eventually sent someone at 4:10 ... if my appointment had been at 4:15 I'd have missed it by the time I got there, but because I knew this would happen, I'd built that buffer time into my request ... if one of my co-workers went running to the manager about my "lie" then I'm fairly certain nobody would be impressed when I told them (and I would be telling them so that they never said anything to the telltale colleague, in case they got caught out the same way )