He wants to give sadaqah (charity) on behalf of a sinful Muslim

If you know people who are not good practising muslims, can you donate money on there behalf? On the day of jugment will these people know that some of there good deeds have come from someone else donating money for them?

Praise be to Allah, and blessings and peace be upon the Messenger of Allah:

Yes, it is permissible for you to give sadaqah on behalf of your Muslim brother even if he commits some sins. We ask Allah to reward you for doing that.

There is no doubt that they will know on the Day of Resurrection that you have given in charity on their behalf.