Free masonry's purpose is to destroy the Catholic church. I would consider it a dangerous cult.
Masonry is very dangerous to Catholics, even in this country. NEVER approach a mason with attempts to re-evangelize them by preaching or apologetics when unsolicited; only discuss if he initiates it; some will, and then be careful. The best way to evangelize them is by good Catholic EXAMPLE; that is, the way you LIVE the Catholic faith.
If you don't know how to approach them at this point, you will quickly be over your head. It's best you just give him some good Catholic material dealing with Masonry and let them deal with the incompatibilities internally. Direct them to discussion with apologists who really know what they are doing and have the time to do it right, like Blackburn, Aiken and Madrid and who do it with charity to their person. A simple but clear explanation of what is wrong with Masonic worship alone is enough to trouble them in conscience, based on the oaths they take, if they are truly well intentioned. If their oaths do not bother them, look out; these men are not to be trifled with!
The links that Andrew offers are good ones to explain the essential differences, thus the real dangers. The implications of their oaths are very, very sobering.
We should be careful what we write, even here.
More probably, he just does not know that a Catholic is prohibited from being a mason. The other problem is that in the US the face of Masonry are the good doing and fun raising Shriners, raising money for many worthwhile charities and causes; the Shriner's circus and children's hospitals.
So the lines get blurred and Catholics, not properly instructed in the faith, fall into that limited vision of Masonry.
So one can not be a Catholic and a Baptist at the same time. Much more so a Mason and a Catholic. I would also point out that while its no longer an ex communicable offense anymore, it has been listed as a grave sin to belong
"The absolute oneness of the Craft", says the Past Grand Master Clifford P. MacCalla (Pennsylvania), "is a glorious thought." "Neither boundaries of States nor vast oceans separate the Masonic Fraternity. Everywhere it is one." "There is no universal church, no universal body of politic; but there is an universal Fraternity, that Freemasonry; and every Brother who is a worthy member, may feel proud of it".  Owing to the solidarity existing between all Masonic bodies and individual Masons, they are all jointly responsible for the evil doings of their fellow-members.
I would guess and would not be surprised at all, from my activities with the Knights of Columbus having to eject some of its own members ,who freely accepted the edict of our priest and didn't seem to care about their Catholocism or the notoriety of excommunication that a high percentage of these very inquiries are in fact full members of parishes. There is only one source that is the Father of all Lies, Chaos and wars' and it is not from within the Holy Trinity!
P.S. 'Jesus, We trust in You!'