I dunno, I guess I see it differently.
My perspective is admittedly influenced by 13+ years of marriage to my first ex husband, who is compelled to (try) to one-up everything. He ate the most, drank the most, spent the most, had the biggest, the newest, the best, and had to top everyone's story. He's also as passive-aggressive as a person can be. He is notorious for spoiling things "accidentally on purpose."
I think it depends on the occasion and the formality of the event. We don't know if this was "an occasion" like the friend's 50th or 60th birthday, or whether the woman went whole-hog and rented a venue, ordered flowers and put down a catering deposit. Some people party that way. If I'd put a lot of time and effort into organizing something like that as a surprise birthday party for my SO and one of his longtime friends let it slip, I'd be more than a little upset. (However, that's not the way we roll.) If we were planning something more like Christmas Dinner or a backyard barbeque, I'd see it much differently.
Depending on who "spilled the beans," I'd also know whether or not that person was trying to cause problems between my SO and me.
If it was an accident, LMT's husband should say "it was an accident" and apologize to the hostess. It may not be a surprise party any longer, but I'm sure the hostess can still enjoy organizing and putting on an amazing party, and the guest of honor and everyone else can still all have a great time.
If, as some have suggested, LMT's husband is aware that his friend does not like surprises, he should have discreetly told the girlfriend directly or had LMT relay the message. He should not have deliberately spoiled the surprise *if* that is in fact what happened. At the very least, it would be rude to the hostess, who is his friend's girlfriend. By being rude to the girlfriend, he is by extension being rude to his friend.
If something like this is an ongoing thing with LMT's husband and this time he's made waves with the hostess, I totally understand why LMT would be upset about it. This may not be a situation covered by "Oh my, what an amusing faux pas." If that's the case, I'd expect him to have a plan for how he's going to make sure this is not going to happen again before he says "sorry."
If it was a genuine mistake and something he rarely does, encourage him to make whatever amends with the hostess as befit the situation and then "let it roll off, like water down a duck's back."
MM
"Our past is a story existing only in our minds. Look, analyze, understand, and forgive. Then, as quickly as possible, chuck it." ~ Marianne Williamson