A friend or another relative to ride with would be much better, IMHO! This is 2017, not the 1950's!
An 8 year old girl, alone, is a target for unwelcome issues. Please think again! Thanks!!!
Also - I believe that she has to be with someone over 14.
Interesting thoughts... most rides are only a few minutes long. There are security cameras EVERYWHERE. Most rides you really can't get your arms or hands into a position to touch the other riders ; if you can, the ride is moving too quickly to do more than brush them with a light touch or a soft grab. if there was an unwelcome issue, all she would have to do is tell the CM's. But warn her that having thighs touch on a moving ride isn't something anyone can do something about, so please do not make some strangers' life a nightmare over physical contact that neither of them have control over.
I have much more issues with the parents who have their children loaded up into strollers, legs everywhere and no undies! AUGH it makes my skin crawl.... hygiene please people! Someone else has to rent that stroller after you!

My family and I spotted at least a half dozen strollers in EPCOT where there was absolutely no concern to children's modesty or health! *siiiiiiiigh* And we're not talking babies in diapers, one family had their 10 year old daughter "airing it all to the world" while she played on her iPad! Let me say for the record, that was possibly the "Quickest Quick-Serve Meal" we ate the entire trip!
*waits for hate*
edit: not to mention, the CM's are really really good at manipulating the seating on rides for single-riders, if they can. And if they can't, they shift around parties until everything was appropriate. I saw the CM's re-direct other parties LOTS of times when we were at WDW.
At the Blizzard Bleach, we as a small party of three, often had to ride with total strangers on the giant family tube ride.... oh what's it called... something Springs... the CM's always paired us with younger girls or other families rather than stick a whole bunch of older guys in with us.
I realize a large portion of party switching must also occur because of weight issues on the ride. If the ride HAS to have a certain minimum weight at the back because it was designed that way, you're always going to be paired with some random single rider or odd member of a party.
But I never had any unwelcome issues with other people at WDW ; everyone seemed a little magically graciously about lines and dealing with each others' nonsense. You are stuck next to these people for the next hour... make it PLEASANT!