Although I sympathize with Suzy's discomfort, it really sounds like a Halloween party, and even being out and about on Halloween, are not good ideas for her. If she chooses to do so anyway, I think it's on her if she sees something--perfectly reasonable for the holiday--that upsets her.
The only thing I would do is mention to Suzy that guests at the party might, in fact, be wearing masks or make-up, since you had previously assured her this was unlikely. Then she can decide if she wants to come or not. Personally I wouldn't offer to text her about the costumes once the party started, if only because someone might arrive later or put on a mask/make-up and I wouldn't want to have given Suzy incorrect information--nor would I want to dictate what my guests wore to that extent.
To address the larger question, To what extent can or should the host of a Halloween Party put parameters around guests costumes?
which I do think is an interesting one... I seem to recall threads here where people wanted to know if they could say something like, "Guests MUST arrive in costume/guests without costumes will be turned away!" or if someone was rude for attending a "costume party" without a costume. I can't remember what the consensus was, though.
I guess my first thought is, if the invitation clearly states it's a costume party, or a "black and white" party, or something else with a specific dress code (not just formal/semi-formal/business casual), then you know
that from the beginning, and if you don't want to dress like that, I think you should decline the invitation and stay home. This is assuming it's just a fun party, and not someone's wedding or some other event of greater significance. It seems a little party-pooper, or attention-getting, to arrive at a costume party with no costume or to wear a bright green dress to a black-and-white party. I think one should think about one's motivation in even attending, if one isn't going to "get in the spirit" of the event.
On the other side, as a host, I just can't see myself refusing to let someone in if they weren't dressed in the spirit of the party. That would be a pretty bold move. Maybe not rude, I'm not sure; but definitely hurtful to the relationship
. I would probably let the person in and then next year, not invite them. Maybe I would keep some easy costume accessories on hand and see if the costume-less guest will at least wear that--hat, cape, boa, glasses with the nose and mustache attached, that kind of thing.
I'm thinking of someone who defiantly says, "I'm not wearing a costume." I consider it okay if someone is in normal clothes but has a line prepared like, "I'm a serial killer. They look just like everyone else." In other words I would consider having a costume to be yes/no--I wouldn't say, "Your costume isn't good enough
for the party." To me that's taking it beyond the "friendly Halloween party" and if you want to have a party where only serious, time-consuming costumes are used, you should make sure your guest list consists of only like-minded people, like a "Halloween costume connoisseur party."
The only other reasonable limit I can think of would be saying, "This is a family-friendly party, so please keep that in mind with your costumes." If the behavior of one of last year's guests prompts you to think of rules like, "No nudity, no racist costumes, no live power tools or real weapons," I'm gonna say, just don't invite that person again.