It's fine, just dont go really fast, and make sure she knows the basics of riding.....like leaning to the inside, leaning forward while going up hills, ect..
I ride with someone on the back all the time with no problems.
lol my new woman was over the other night and we had a break from this freezing weather all winter long ..it was maybe 55 out and i pulled the 450 out and told her 2 jump on...shes like omg its 2am..im like f-it were gonna go 4 a small ride...so i ripped about 2 miles down the street and then came back. Alot of wheelier were involved. it was her first time on a quad so shes was a lil w00tered up after that ride :lol :woo