When writing will, adult child is deceased, so his share is to go with the surviving adult siblings. Wife of deceased child adopted a grandchild after his death. She claims the child is entitled to deceased share from his father. Is this true?

Depends on how the will is exactly worded,

If this is a will you are writing, thenyou can include them if you want, but you don't have to.

As Chuck noted it depends on the wording. If the will specifically states something like:

If any of my children predecease me, their share is to be distributed among the surviving children.

Or

My estate is to be distributed evenly among my surviving children

Either way the grandchild gets nothing. However, if the will says

The estate is to be divided evenly among my children and their offspring, then yes a grandchild would have a claim.

Wait a minute I just re read the OP. You state the spouse adopted a child AFTER the testator's child died? Then no, that child has NO claim on the estate, since the testator's child never adopted the child. Unless the adopted child was specifically mentioned in the will.