Hi Sojuu, welcome to BikeForums.

I really doubt that you will be able to mount smaller wheels. This bike appears to have v-brakes, which since they are mounted to fixed studs, will likely not match up with a smaller rim. There are drop bolts available, but I don't think they work for all applications. If the bike has knobby mountain bike tires, you may be able to cut a fraction of an inch by moving to smoother tires.

I'm assuming that you want smaller wheels because your wife has trouble planting her feet when stopped. You've probably attempted to lower the seat and that's about the only adjustment you can do. Perhaps if your wife can put at least one foot down she can gain some confidence and continue to use the bike, though I know it's hard to find enjoyment in a new hobby if the equipment is not suitable. My recommendation is to strongly consider purchasing a different bike. The Del Sol is rather low end bike and you would be better investing money in a more suitable bike than spending it on upgrades that may not work.

Good luck to you.