In the spirit of the "Not" jokes that have recently resurfaced in "Borat", a MILFN is a less than hot ass mom of a friend's that you really don't want to do.
You mean the one over there?
Yeah, I wouldn't touch her with a ten foot pole. She's a total MILFN!
n) "Let's go buy your Mom's friends some drinks and see what happens. I could use a good milf'n."