I got out for dinner a lot, so I know this problem well. What I do is eat very light during the day. Then eat a 'healthy' ish choice on the menu. For example, a chicken dish or a tomato pasta dish. Most restaurants do spagetti bolognese so I usually go for that

Usually the only things I avoid are creamy sauces like carbonara or cheese. And if there's a chocolate fudge on the dessert menu I can't resist

However, I think it's important to remember that you only live once, you're going to go to a restaurant and have a nice night out and enjoy it. Dieting is important but we can't be sitting in twiddling our thumbs as major social events like going to social dinners pass us by. Yeah it might make us that smidgen of a little bit slimmer not to go, but whats the point of getting skinny, then emerging to find you've got no friends left because you made yourself a social recluse in the process of getting there? Dieting IS important, but It can't rule our lives 24/7. Without sounding self-indulgent, this is the motto I've had and I've lost 60lb in 5 months , still got all my friends and still have a bloody good time when social times call - so there must be an element of truth in it.
But beware, if you step on the scales you'll see a bit of a water gain - just drink plenty of water for the next days, go easy and you will come back down to normal within 24 hours.
Hope you have a lovely night!