All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Sharee'ah takes all precautions in the matter of approving one's lineage. If there is any possibility even if it is weak to prove someone's lineage to the one who is married to his/her mother, then the Sharee'ah considers it and approves it.
Therefore, the pregnancy of the divorcee of your friend is considered from him according to the Sharee'ah. He is the father of that child and he is responsible for it. The Prophet said: “The child is for the owner of the bed and stoning is for the person who commits illegal sexual intercourse.” [Al-Bukhari and Muslim]
The above ruling is put into effect as long as it is not confirmed that the child does not belong to him. If it is confirmed that the child is not from him, he has to deny it by performing Li'aan.
For more benefit on Li'aan, please refer to Fataawa 193277, 428203, 122944, 176757, 200749, 88978, 90098 and 84301.
Therefore, you should convince your friend that the mentioned daughter belongs to him according to the Sharee'ah; she traces back to him and inherits from him, and he inherits from her too.
Also, he is responsible for her; therefore, he should take care of her and spend on her.
Allah knows best.