Despite Tillman, 34, having more experience, Roxaine said she was confident SBW had the skills and heart to come out on top of his older and heavier opponent.
"It's all up to [SBW] really. If he has done the training and worked hard he can do it. He's just got to believe in himself and listen to his coaches."
Roxaine and Aroha, who is currently away on holiday, will be joined by their boxing coaches Aaron Warren and Lee Edmond. Both men agreed with Roxaine's assessment.
"It's a good test for [SBW] especially an American fighter. In America professional boxing is at the top level. So this opponent will be no slouch. I think he can win it on points if he sticks to his game plan," Warren said.
Edmonds said he wasn't too concerned if SBW didn't knock out Tillman.
"The crowd always applauds a knockout. That's what everybody goes there for. But I believe if Sonny wants to get the best out of what he is doing, I actually hope it goes the full distance."
It was important for Williams to win the fight, Warren said because it would give the rugby star some much deserved respectability within boxing circles.
"I think because the other guy is from America, he is a totally different kettle of fish. So if he knocks the guy out, all the promoters will get [excited] and say 'well, Sonny Bill might just deserve to jump a couple of scales up the rankings', because it's all about rankings in professional boxing."