It's pretty normal for kids to give quick pecks and even smooches on the lips especially if they have affectionate families. And kids do it to friends sometimes, until about he age when the other sex has "cooties" which differs for different kids. The extensive kissiing you describe is disturbing, yes, but if you give it all a huge emphasis she will too.
Talk of boyfriends and girlfriends seems to come up early (and in kindergarten I had a "boyfriend" but no, kissing would not have entered our heads). If she has been talking boyfriend talk a lot prior to this, or you notice she talks or plays a lot about having boyfriends, I'd be sure to examine what videos or TV she is watching (is it too mature for her? Much of Disney channel stuff is for preteens, not litle kids!), what her girl friends play and talk about, etc. And change it. Even "nice" animated movies like "Tangled" usually have some kiss involved when the hero and heroine finally lock their googly eyes on each other.
I would just tell her matter-of-factly that she can kiss mom and dad (and grandparents or whoever else you put on a list) and but save hugs only for friends. Blame it on colds and flu if you must! And you can tell her that it's nice to have boys for friends but not boyfriends, I guess.. But I would not overemphasize all this, or get upset in front of her, forbid her ever to see any boy as a playmate, or give her a big lecture. She will be so confused and think she's done something horribly wrong --which could have one of two effects for a kid her age: She will be upset and angry with herself and get very sad and guilty and clingy because she upset Mom; or she will get defensive and possibly seek this boy out to kiss him again, just to prove she's her own person. She and he very, very likely do not have any idea of real sexuality and these kisses are mimicking adult behavior or behavior they've seen on TV or wherever. (Unless one of them has witnessed inappropriate behavior or been abused themselves but you have no knowledge of that in the boy's case, do you?) But if you go too crazy explaining and lecturing she will see that you place some very high "value" -- even a negative value -- on what she did and that will make it loom very large in her mind. Do you want her super-focused on it, thinking about it even more, or directed to keep kisses to herself and leave it at that for now? Of course I'd keep a close eye on her with any boys who come over.
This is also the age to be certain she understands appropriate touch -- only mom, dad and the doctor can see you or touch you in your "bikini areas," etc. Get some kids books about this geared for her age - there are plenty. Don't associate it with this boy or with this incident, though. She will be very confused if you act as if he did something like that to her.