To bring a gift?

I’d take a small gift and offer to pay for a drink.

Surely he should offer to go halves with the meal? Why does he think she’ll pay for the food?
Maybe take her flowers?

I agree with Cossy

Offer to contribute towards the food and drink and take a bunch of flowers.

Most certainly a bunch of flowers and then see what opportunity arises to pay for something at the meal.

I would take a gift and offer to split the bill.

Don't take flowers. They'd have to left to wilt in the car as you're eating at a restaurant. A small gift (possibly a memento of their shared past? maybe a family photo?) plus splitting the bill should be fine.

Her husband will probably insist on paying as they have invited him. I will look out a small gift and he can pay for drinks.

I would offer to go halves on the meal.

Just split the bill - flowers would probably wilt but he could consider posting flowers to her afterwards

They have invited him. I don’t think he should pay for their meal. Yes, take a little gift for his step sister, and buy a round of drinks, if there aren’t too many of them. If not, just a gift and a drink for her. I’m sure he’ll have a lovely time.

I don’t think it shoukd be assumed that your dh step sister and husband will pay. I would split the bill if it was just them, but if there are others from her family coming too, for a birthday say, then definitely send a bouquet after the event or take a gift or both.

I think she is looking up some of her cousins so there may be a crowd.My son has offered to drive him .He loves meeting new people.