Unless you have a written agreement (such as a lease) that specifically states your tenant cannot have overnight guests, then I wouldn't press the issue--not for a one-time incident. Talk to her before taking action. Let her know that this is not an acceptable thing in your home. And when she has overnight guests, they are not to be given access to your part of the house. Then politely tell her AND her guest that they need to ASK you before spending the night. And that your couch is off limits. You own the house, you make the rules--but you need to make sure your tenant KNOWS the rules first.

she is a friend, renting a room, we had a common sense agreement,

but last night she arrived back after being in a fight. And she let the guy that brought her back sleep on the couch. Its now 12pm the next day and he is still asleep on the couch.

Not to mention the guy previously tried to start a fight with me a year ago when he was drunk.

Can I go down and kick him out?

If its your house then yes you have every right to make him leave. You then need to sit down with her & make perfectly clear her guests are not to occupy the common area. She is renting her room & her room alone.

As long as the people don't stay longer than stipulated in the rental agreement/lease....it's fine.

Just tell her not to do this again as it puts others in the house at risk.

Try this question in Friends and family section!

You will get more answers than you will in the "dog section."

You can go down and kick him out. You can tell her that it's not acceptable.

It is your house - you can tell him to leave. Don't kick him - that would count as assault.