Mundine watched his mate go through his paces during an afternoon training session with his new teammates at Rugby Club Toulon.
After about two hours sweating it out on the field in blistering conditions, the 23-year-old New Zealand international left the training ground with Mundine in a club car.
Hundreds of fans turned up to catch a glimpse of Toulon's controversial new recruit, who happily signed autographs and posed for pictures before saying "au revoir" in his best French.
Williams ignored questions from waiting reporters, while Mundine was tight-lipped about his reasons for visiting his friend on the Cote d'Azur.
The boxer, who like Williams used to play rugby league before switching sports and has publicly supported his decision to flee the Bulldogs, said he planned to spend at least a few days in France.
When asked if he would attend the Top 14 club's first pre-season friendly match tomorrow, Mundine said "probably".
The Muslim boxer is believed to be one of the few people who knew about Williams' decision to walk out on the Bulldogs last month despite being just one year into his five-year contract with the Sydney club.
And in his first interview since fleeing Australia, Williams has revealed that one of his reasons for leaving was because of how his former coach Steve Folkes questioned his friendship with Mundine.
"I rock up to training and Folkesy, Steve Folkes, someone, that to be honest, has never paid any interest in my personal life, he comes up to me and starts saying: `You're not turning Muslim are you?'," Williams told Channel Nine's Footy Show, according to extracts published in News Limited newspapers.
"I just laughed. I said: 'Seriously you are joking.' And then he has a go at Anthony: `You're kidding yourself if you take advice off him.'
"I start to think: `Who is he to question my friends?"' It is not yet known whether Williams will play in tomorrow's match against third division team Hyeres, with team officials expected to make a decision later today.
Toulon's team manager Tom Whitford said while Williams still had to master the finer points of rugby, he had made a good initial impression and could have a role as an outside centre or on the wing.
"We're hoping that he will play," Whitford told AAP.
"We'll make a decision probably tomorrow, not just for him but for everybody who's going to play.
"We're going to try to get as many people to play as possible. It's a good opportunity to give everybody a go to see what they're worth." Whitford said neither he nor his players were being distracted by the ongoing Australian legal dramas involving the Bulldogs and NRL's attempts to stop Williams playing rugby in France.
"We're not thinking about that," Whitford said.
"We just want things to work out for his sake and for our sake because obviously it's in his interest to play well and our interest as well.
"We're just going to look after the present really and what happens happens, we can't control that." Also unperturbed about off-field dramas were Phil Croce and his 10-year-old son Antoine, who were among the lucky few Toulon fans for whom Williams signed autographs.
"Sonny Bill Williams is a star in Australia and will be in France," Phil Croce said.
