Well that wouldn't be very nice would it?..I mean what makes u so sure ur unmarried gf's that fond of all ur husbands and yet she never says ... hey why don't we exclude the men or children from our get together?..
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If you're going to invite her I don't see how u can say "and btw please don't bring that skezball of a bf with you"..
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Unfortunately, this is who your friend has chosen to go out with, so you'll have to find a way to get thru the evening ... for her sake...
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I don't imagine this will be a long term problem by the sounds of ur comment "we avoid socializing with him/them whenever possible"..if they continue dating neither of them will be on the invite list next year.....
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Sad tho isn't it ... I can't imagine how much she was valued as part of the gang ... when she's so easily disposed of based on her choice in men...

If you want your friend to be "part of the family," you have to suck it up and invite the jerky, skeevy boyfriend. That's what families do. They don't exclude people for subjective reasons. They get together and have miserable, awkward, squirmy holiday parties. In order to be polite, and to support your friend (because that's what *friends* do), you'll have to invite her other half, even if you don't think he's her better half...

have any of you actually talked to your friend and explained the problem.. no need to tell you don’t like him.. ..
Alternately ..
may be you can say that your friend is a different person with him around,,, and you guys miss your old friend?..