The Italian manager say he did not stand in the way of the move while also thanking Williams for helping him out in his first season in the Premier League.
"I want to say something about Ash. He was very important for me when I arrived for my first experience of the Premier League," Guidolin said.
"I would like to thank him for the help he gave me. He was a real captain and I respect his decision to leave for a new challenge.
"When a player wants a new challenge, it is correct to respect his decision. I knew there was interest from Everton but I decided to stay out.
"I believe when a player decides to go, it's better to let him go."
Wales captain Williams left the Liberty Stadium for Everton in a £12million deal this week having spent the past eight years in South Wales.