I recently purchased a guitar from a friend, but however im not sure what the exact guage is, and the previous owner says she can't remember either. She does know they are probably .09's. Would putting a set of .10's have an adverse effect on my guitar?
No. It may need intonation and a truss rod adjustment though, maybe raise the action a bit.
not much would happen a setup might be in order but considering the last owner didnt even know the gauge you might want to get one anyway