Try rubbing ice over it, it works for gum, maybe it will work on Silly Putty.
Put some dawn dish soap on it let it sit for a while and then hand wash.
Dampen the area and put the shirt in the freezer until the dampened area is frozen [the dampness is just a gauge to tell if the silly putty is frozen] Pull the shirt out of the freezer. Manipulate the material. The silly putty will shatter off.
If you freeze it, then try peeling it off no doubt that should work
Try ice! Ice the opposite side of the shirt where the silly putty is and hopefully it should come off!
Try rubbing salt into the silly putty, let sit overnight and then it should come out very easily.
You could probably wash it and see what happens!
Freeze it and it will crack off
Try taking ice and rub it on the shirt it should peel right off
I'm not an expert but run it under water then rub ice in it if it is still not gone