Voluntarily helping another, ie charity, is entirely compatible with libertarianism. If you ask us "Are you your brother's keeper?" We say, "Yes, when we want to be."
If we are denied that choice, the question is not "Are you your brother's keeper?". It becomes "Are you your brother's slave?"
There is an argument for this. If the public didn't provide a safety net that job would fall to churches and civic organizations. Churches would then be able to minister to the soul while they are providing whatever assistance is needed. Churches would rise back to prominance and only churches filled with people who really care, who are willing to give would flourish.
In that respect it could make for a much better society. Of course, I wonder how many of the libertarians would suddenly see value in the public dole if the alternative was turning to the church.