This is, on its face, illegal. But there are a lot of other things to consider.
1. Is the person seriously trying to convince others that she is your friend?
2. Even if she did convince some people that she is this artist, what would that mean?
3. What, if any damages result from this? (i.e. is she selling your friends pictures as her own)
4. What about the subject material? If it is already copyrighted (i.e. she's a Naruto fan artist) the rights of ownership become muddied, technically the person is stealing a stolen IP, which opens up a whole new kettle.
Your best bet is to contact the site administrator with your concerns and ask them to shut the person down for violating terms of service (this is something that will be in the terms of service, but you might want to review them and quote the section)
A cease and desist order will mean out of pocket legal fees, and you would have to issue said order to the site anyway.
If there are no damages attached to this case, you simply don't have a civil case. And it is unlikely, given the details you've provided, any government agency would waste time pursuing this through criminal channels.
Maybe if you found out the location of this person, and it is nearby. Even then it seems like it's more trouble than it is worth (for those of us who aren't rich anyway)