We have just recenrtly tracked down my kids' Nan's missing uncle, well his children, the uncle was taken away by the catholic Nuns in 1911 after his parents both died within months of one another, and the 3 older children were out at work and one day came home and the little bot and his baby sister had been taken. Anyway, those two children were put into Southall Orphanage and the girl became a Nun and the boy was put into the army as a boy soldier. He worked at the Woolwich Arsenal and then was transferred to the starfordshire Arsenal in 1938 and married locally, he had 2 children and we have now contacted his surviving son who had no idea of any of us , the Uncle died when his son was 15 and so this cousin has not had the chance to ask anything of his dad. So now we have made contact purely by confirning the civil registration records were correct and writing to the address on the Uncles daughter's death cert, which was of her brother, the surviving cousin.
So we wrote to him last week, explaining who we were, how we had found him, explaining how the family was connected and we gave him the option of telephone, letter or email, and on sunday night he emailed us, he was absolutely over the moon! He had no idea he had such a huge missing family!!!
To be honest until the 1911 census came our neither did we, as the generation his father belonged to did not talk about what happened to the 2 children, nor did they have the ability to track those children down,
My advice to your friend would be to do a personal letter, explaining who she is and that she has always wondered who her siblings might be.
Good luck to her I say, nothing ventured nothing gained.