Stowe co-starred with the Oscar-winning filmmaker in the 2002 big-screen drama "We Were Soldiers."
However, Gibson's career has been overshadowed in recent years by his battle with alcoholism and legal troubles.
The "Lethal Weapon" and "Braveheart" star grabbed headlines in 2006 for making anti-Semitic remarks during a drunken driving arrest, but later apologized for his actions. His public image was further tarnished when audio apparently of him making racist and abusive remarks toward his ex-girlfriend Oksana Grigorieva were leaked on the Internet.
Grigorieva also accused him of assaulting her. The actor denied any wrongdoing, but pleaded no contest to a domestic violence charge and was placed on probation in March 2011.
Asked in an interview to air on Wednesday's edition of "Katie" what she thinks happened to Gibson, Stowe replied: "It's not for me to assess what has been going on in his heart and in his mind.
"I can only tell you the Mel that I know, and that Mel is the same one other people [who] have worked with him know. He was probably one of the most wonderful people on the set ever, that you could ever imagine," Stowe said. "A good friend, he does tremendous humanitarian work that he has never told anybody about. He and his ex-wife Robin started a foundation called Mending Kids International. They do heart surgeries; they perform life-saving surgeries to impoverished children all over the world and have never garnered publicity for this. He has a wide variety of interests. He has been a very good friend to me. He is very tortured by what's going on. I do not believe he's an anti-Semite in my heart of hearts. I think that he's a very complex, complex, deep individual, and I think Jodie Foster has remained his friend. I have remained his friend and I hope the best for him. He's a real creative force, he's a special man."
Stowe can now be seen in the hit TV series "Revenge."