It's illegal if done without consent; if the player is willing to risk being dropped into the void, then it's not our place to deny that.
Honestly, if the player agrees, just screenshot the chat when they do. So if they choose to report you, with their negligence, you have something to back up that they did indeed agree to it.
One thing I am going to add it some players may just visit you again while trusted and grief you so be careful!
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