One of my bridesmaids is in an on-again, off-again relationship. I know the guy's name, but I don't consider him a friend. Assuming they're still together by the time the wedding rolls around, I realize he should be invited, since she definitely considers him to be her significant other when they're together. But if they break up, I don't want to have sent him his own invite and have him show up! (They don't live together, so they'd each have their own.) Would it be way too tacky to invite my bridesmaid "and guest"?
You're already trying to do the right thing, so just run it by your friend to ensure no feelings are hurt. Bridesmaids are used to brides confiding in them, so broach the topic by bringing up guest-list gripes. Then, without hinting at the rocky nature of their relationship, say, "Hey, should I invite you plus a guest or send Jake his own invitation?" Her response will tell you exactly how to handle it.
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