my friend bought some themes in themeforest site about 99$.can i use that themes by taking themes from him.any problem will occur in future
you could try to contact the author and ask if you can use it on 2 websites. maybe even pay a small fee for it rather than re-buying the theme. But if your friend is not using one of those themes than you probably could use it.
You can also get the Themeforest themes to use on multiple sites.. if he purchase like that then you can use that otherwise.. you have to purchase in that manners.
It is illegal. Instead, try to use hueman or anew from (alxmedia DOT se). These are completely free and highly customisable.