Can a person testify for his adopted child? According to R Yehudah in the mishna, I think he'd be classified as a "ohavo" a loved one. But we dont paskun like R Yehuda.
Maybe he's like "your wife's son"?
But halachcally, an adopted child is not related.
Tzvi Stern, RBS
The Chachamim hold that one is not Chashud to lie even for one he loves. Therefore no matter how great the love of the Ed for the defendant, he is allowed to testify. A Karov, on the other hand, is Pasul only because of a Gezeiras ha'Kasuv (Bava Basra 159a), which obviously does not apply by an adopted son.
However, if the Dayan sees that the witness stands to benefit monetarily from his testimony he may disqualify the witness as a "Noge'a b'Davar."