Q: Can I testify on behalf of my partner or former partner?
A: A person with a vested interest — nogei’a badavar — cannot testify; however, once he is divested of interest, he can testify (C.M. 33:15).
Thus, while you are a partner, you cannot testify if you will gain from the testimony. However, you may testify if the testimony will not impact you, or is detrimental. Similarly, you may testify on issues unrelated to the partnership, where you have no vested interest (C.M. 37:6).
After you leave the partnership and are divested of interest, you can testify. According to most authorities, you can testify even about events that occurred while you were a partner. (This differs from inherently disqualified witnesses, such as relatives and thieves, who cannot testify on past events even after becoming qualified.) Some disagree with this, so that if the other party is in possession, he can claim kim li and refuse your testimony about events during the partnership (C.M. 37:6; Shach 37:32; Pischei Teshuvah 33:8).