No. One person can’t do teshuva for someone else. Part of the teshuva process is for the sinner to decide they will (try to) never do the sin again. AFAIK, it is impossible for one person to decide that somebody else will never do something again. It is also impossible (AFAIK) to do teshuva after a soul departs.
However, if the child of the niftar does teshuva then that is a big zechus for the parent – even if they have already passed on.
Mashiach agent is trying to say that he would like to inspire all of the Jews in the world to do teshuva by his acts of tzidkus – but he can’t do it for them.
You probably would get a better answer from your local orthodox Rabbi.
Kesiva V’chasima Tova!