If I understand correctly, your husband allowed her to have a boyfriend last year, but this year he has a problem, perhaps because the boy is 18 and has his license. Maybe last year, your husband was driving her and a younger teen to a movie or a party?
I suspect your husband is realizing that his daughter is maturing - physically and emotionally. Perhaps he remembers what his friends (and maybe he himself) were like at 18 - plenty of talk about taking advantage of a younger girl. But here's the fallacy in that: a car is an issue for driving experience but not for sexual activity, an 18 year old guy is probably on a similar maturity level to a 16 year old girl, it's easier for a somewhat socially awkward guy to spend time with a less experienced dating partner.
So either your husband or the girl's mother or you - hopefully in cooperation - have already adequately prepared this teen for life "out there" in the teen dating world. You've had the sex/disease talk many times, you've had the "respect yourself" and "no means no" talk, and you've encouraged her to choose wisely in all facets of her life.
This boy is a senior and will be heading elsewhere in 6 months. This is not a long-term relationship, most likely. In 2 years, your stepdaughter will be heading off on her own - perhaps to college. Exactly when is she supposed to magically gain life skills and dating skills, if she doesn't "practice" now while she's still under your roof? The surest way to get her to rebel and sneak off is to prevent reasonable dating now. You've met the boy, he's been willing to come to your house and face your husband, so you've screened him a bit. Give him (and her) a chance. I'm assuming she has a phone, that your husband has put some rules in place (including checking her photos and texts periodically) and that he has the ability to track her location (and that she knows this). She's not asking to spend a weekend in a remote mountain cabin, right? She wants to go to the movies or out for pizza or to the Homecoming dance. Let her.
Your husband cannot protect her by saying no to everything. Either he's done his job as a parent, or he hasn't.