Boanas - now the manager of Estonia Women - coached Williams at Charlton during the formative years of the midfielder's career.
And the 55-year-old praised Williams ahead of England's friendly with Sweden on Sunday when she is expected to win her 130th cap and become the most capped player in England history.
"I remember clearly her first training session when she joined us from Chelsea," Boanas told Sky Sports.
"She simply oozed class, a technically wonderful player to watch and so focused when you coached her.
"Rather than supremely, I'd say she was quietly confident in her own ability, calm and calculated on the ball with a desire and ability to learn from each experience and each game.
"She had a little bit of mischief in her personality but was always humble and respectful of others and combined that with a hunger to win making her a joy to coach and work with."
FIFA Pro Licence holder Boanas is one of the most respected coaches in the women's game and has also previously managed England internationals including Casey Stoney, Eni Aluko and Katie Chapman.
He paid tribute to Williams ahead of her prospective milestone, revealing his pride at having had the opportunity to work with her.
"It's an amazing achievement for any player to top the international appearance lists," Boanas added.
"She will be one of the first to admit it has been made easier due to the vastly increased amount of fixtures.
"To have remained in the frame for selection is tough enough but to be a starting player - especially as the competition increases - is testament to her adaptability and passion for the game.
"Knowing some of her background it is, I am 100 per cent certain, an extremely proud and emotional achievement for her personally.
"I would like to congratulate her. I'm very proud of her and proud to have been part of her journey."