What is it? Growing in my friend's yard. Very cool, but she definitely didn't plant it, and we're not sure if it is a weed or not. Any ideas?

UPDATE: I found the name of the plant. It is "Venice Mallow" or "Flower of an Hour" and is considered an incredibly invasive weed. Apparently the seeds can lie dormant for 50 years! It has a really pretty white flower with a purple middle. I called my friend and told her to get rid of it ASAP.