After she gets used to the feeling of something being in her ass you can slowly work another lubed up finger into her to stretch the hole further.
Slowly, and I mean slowly, work your tool into her ass. Once it's in there, just rest for a time to let her get used to it. Believe me, once she does get used to something being in her ass, you can pound the hell out it and she will have one of the hardest orgasms she has ever had. Be warned....once in a while, when you are finished and pull your tool back out, it might have a little "stuff" on it. If you think you can't handle it, don't go there. Consider where you're putting your dick.