My sister has been in contact with the newly identified half-sibling, who never had an inkling that their dad was not the biological father.
The only advice I would have given is already water under the bridge.
I would have advised not to contact this new sibling if there was a surviving parent. (assuming that you could find out).
I would have advised against possibly alienating the affections of the surviving parent and child over a long past infidelity.
My sister had no idea that I would immediately know who this family and half-sibling were, due to where I live, and having had contact with the family in the interim years due to strange coincidences.
Then also, having found out that the person lived right near me, kind of gives me the feeling of God putting people in the same place at the same time, in an effort to be sure they meet up. (I have had several experiences in my life like this, which have been positive, but kind of eerie.)
It is up to the new half-sibling about whether to move forward now, it seems. They have a great deal more of new info to absorb, than we do, as well as to decide about asking the mother. Also about telling their own siblings, children, etc.