as a big user of facebook i'd just like to say that on the "how you met" section there aren't a huge amount of possiblities.
however, there is "met randomly" which would probably have been more appropriate in this scenario. there's also "worked together" altho that probably doesn't fit quite right here either.
when you state how you met the other person has to approve this information but tbh, in a facebook context i wouldn't read too much into "we hooked up" as an indication of anything untoward.
given the details shared about this woman and the way your dh met her i'd be inclined to think he knew full well you'd be unhappy with it therefore didn't mention it. especially if he was doing it for a mate altho if you're suspiciously inclined that could be read as being the oldest story in the book! the mythical friend!
anyways, on the whole the fact that she's on his msn and therefore sucked in thru the facebook friendfinder and that he hasn't hidden or removed her details etc could point to the fact that there really isn't anything to hide.
your dh clearly thought as far as how this would affect you by deciding not to mention it. but, as is often the case, he didn't think any further than that to the point where he probably should've ignored the friend request and not bothered with her any further.
as for sending beers, it's the facebook booze mail or happy hour and you generally end up sending them to everyone on your friends list. it's like any of the applications you add, the more friends you invite the more credits you get etc.
see, told you i was a big facebook user lol!
only you know your own instincts on this one but if it were me, i'd try to remain calm and reasonable. tell your dh that hiding things will only fuel suspicion and distrust and that it's probably not particularly appropriate for him to encourage anything other than an aquaintance type relationship with her.