The trouble with that though, is that a weekend happy hacker might decide they want to start showing, and suddenly their horse isn't suitable for what they want to do. So what do they do then? Sell to another weekend happy hacker? Send the horse through auction because they can't sell it? Keep it and attempt to make it into something it isn't capable of being? It's a story repeated all too often in my area. The only reputable horse rescue in my area is run off their feet with the unwanted horses that are only sound enough, or only educated enough, for the weekend happy hacker. I know quite a few people who only ride at walk, and would be quite satisfied with a horse that can stand up to long periods of light work at intermittent periods. The problem comes when those same riders get more confident and start wanting to trot and canter, jump, show... and their horse isn't suitable. That's when they break down. That's when they go for meat.
I believe you should only take on a horse with bad legs if you are willing to keep it only in light work, and if you are willing to take on the risk that it may become unsound in very short order - and you are willing to keep it anyway, or else put it to sleep so that it doesn't suffer neglect in its lifetime. It's the unsound that are neglected far more often than the sound.
OP's friend can do whatever they want. My opinion is, don't buy this horse. Call me callous, but some horses are destined for the meat pen, and sometimes, rescuing them is only postponing the inevitable. Others, of course, end up there by mistake or through humans screwing them up - but that's another issue altogether.
What REALLY bothers me about this one is that she is a filly and therefore someone out there might look at her and think, hey, it has a uterus, let's breed from it! One of the many many reasons I dislike the fact that here in Aus, you can't spay a horse.