It is not "bad" for the tank but unless something is eating it, it will completely take over. You can't pull it off because their roots are as fine as hair and all over the stem. If your friend can, have him get it out of his tank. It will clog the powerheads, filter, it gets everywhere!!
Maybe remove the whole rock if it's a small one. I put some in my tank when I was new to the hobby (LFS advice
) when it started getting all over everything, I put in a coral beauty and he took care of all of it. I was very lucky.