I go through Tinder for a chuckle sometimes, and I get amazed at the number of duck face white girls who are apparently browner than me. Not sure if it's a london thing, but I didn't think they were actually much of a thing anymore.
I basically use Tinder to judge the world, which is essentially its core focus. WE do make snap judgements whether we like to admit it or not, Tinder just has no opportunity for anything else if you swiped the other way.
I'd say re-arrange and then stand HER up, but that'll probably be difficult as she won't be there :p
I've generally been of the same opinion as yourself(albeit situation depending) - she's in the committed relationship not you, and it's up to them to sort that out between themselves. People feel very strongly on the issue though, I've found. The counter is - what if I was in his shoes etc - yeah, I'd be super pissed off but(again, situation dependant) the 'other guy' didn't make any sort of promise or commitment to me.
However! You know you're doing something you shouldn't be, and don't be outraged at any potential consequences. You might catch a smack or worse, and you have to ask yourself - did you truly not see it coming? You're running risks, and they come with their downsides - you do have to consider that it's going to be messy if anything even comes of it too, so it may not be worth the real hassle of it all.