If you were Melanie in this situation, would that be the last straw? In less fantastic situations, would someone putting you in physical danger end your willingness to continue the relationship? Does it make it easier to forgive if, like Ivy, the person doesn't have full understanding of the situation?
Okay but seriously, I'm pretty sure Mennie thought it was all fun and magical games until she realized how powerless she truly was. There's a good reason why so many movies and TV shows use a calm child's voice saying something akin to, "we want to play with you... for-ev-errrrrr," as a major creepy thing.
Now in less fantastic situations it would depend on if the person put me in danger while being stupid or a jackass and if I saw the danger coming, warned them, and they chose to ignore me and screw me or us both over. I just really have no tolerance for super, abject stupidity. This is why I have never liked or tried to stay friends with "frat boy" and "party girls" types.
Finally, if there was hope of making the understand what they did was wrong and proper etiquette to avoid it happening again then yes, forgiveness would be much easier because a mistake is different if someone learns from it because there is a much less chance of it being repeated.
Were I the same age, I'd probably not come back. 'course, I had a decent family environment to return to, wherein Melanie could possibly find even the dangers of dealing with Ivy preferable to that of dealing with her parent(s). Actually, re: stupid kid antics/behavior, the Soup series by Robert Newton Peck tells perfectly of an example of friendship that's maintained because of the dangers they get up to instead of in spite of. The backwoods of the early 20th century were their 'Nam, per se^^; re: last query, I think I'd be more likely to run because she didn't know better. One misstep could leave you a pink smear in another county, or be put in a casket toes up/face down 'cause she caught an iconic scene from The Exorcist.