I’ve had clients who don’t want to open a Team plan and share their login with me instead, in the past. It works (so long as they don’t login to Webflow at the same time and it kicks me out – annoying). But I much prefer the Team method.
It’s not against Webflow ToS to give him your username/password to your main account but that comes with a lot of risks: Not only could he delete all of your sites, but he could also bill hundreds of dollars worth of Webflow services (hosting, Team plan with 20 members, etc) to your account. (which you couldn’t chargeback through your credit card because “you” authorized them by sharing your login).
Technically the Team plan is the only reasonable option here. Then you can control exactly what sites the developer accesses and also see when he is actively publishing the website.