I realize Friedman was a part of the Reagan Administration, so even if he disapproved of supply-side, he may have moderated his remarks, but what kind of things did he say about it, and what can be determined about his real opinion?
Friedman's involvement with the Reagan administration was only as an advisor, he was not part of the administration as a cabinet member or on the budget committee (roles he was offered), specifically because he did not want to moderate his writing and public opinion.
On the Reagan administration Friedman supported the Reagan Era tax cuts, but noted that tax cuts must be coupled with government cuts as well. In fact he even said that cutting the size of government must come before tax cuts, and predicted correctly the increase in he deficit. It should also be noted that Reagan himself was strongly in favor of government cuts in areas of education, agriculture, and environment, but he had to pick his battles with the congress controlled by the democrats (this is outlined very well in Steven Hayward's book "The Age of Reagan: The Conservative Counterrevolution").
Friedman also once famously said on Uncommon Knowledge with Peter Robinson "There is no such thing as supply side economics, nor demand side economics, just economics. You do not increase production by taking from one and giving it to another, whether that be from supplier to demander or vise versa". It should be noted this was after the Reagan Administration.
Generally however, Friedman was a strong supporter of the Reagan administration, because he thought that this was the first successful political movement in over a century to embrace the founding principles of America and liberty (he had the same view of Goldwater's 64' campaign which was unsuccessful).