There are many options for a pre-wedding dinner that don't have to include going to a restaurant. If you need a place bigger than your home, then try the church where the couple is getting married or your own.
Many churches have kitchens and halls that can be used for next to nothing.
Or if you want to chance the weather, since it'll be summer, try your backyard. My friend had to host one last year and they rented a couple of canopies and got tables and chairs from a church. I have also 'rented' chairs from our local funeral home for free - they do it as a community service. Inexpensive decorations included balloons in the wedding colors and tulle gathered down the center of the tables with some ribbon in matching colors. They made all the food themselves- salads and desserts a couple of days in advance and then had a cookout that day.