Don't tell your daughter what you think.  It will just push her into his arms faster.

Next, stop worrying if you are cool or not cool.  You, like myself, are a dad.  That means our kids will never think we are cool (except for some odd moments) and they are not our friends.  They are our children.

So I am hoping that it is a general sense of you do not like this guy.  If it is anything else (drugs, beatings, anything that is criminal and will harm your daughter) there is different advice for it.

So there isn't much that you could do. Be nice, be open, be civil and be warm.  But still be on guard.

The one part that you can do though is this:  if for some reason they are shacking up, or getting an apartment together, then they are adults who wish to be co-habitating or married.  So don't send any money, do their laundry, take care of their things if they are going to live together.  If someone wants to act like an adult, they have to be an adult.