During spring break, it will be hard to find any cruise that does not have quite a few young people on it. Unless you go very high end. But, there will still be plenty of others in your age group. Average age on Carnival, for example is around 45, if I remember right.
The first night, there will be a singles mixer. Somtimes, there are 2, one for the young-uns and one for mature people. Your friend should definitely check those out, since it is where singles tend to identify each other. You can also go to the maitre d' as soon as you board and ask to be seated wiith others traveling solo.
I cruised a lot when single (and after I wasn't!) and cruises are about the easiest way to meet other singles. You meet someone you might be interested in and you don't have to plan a "date". you can just agree to meet for an activity or a cup of coffee on the ship. If it doesn't work out, the ships are definitely big enough to avoid each other.....I know this for sure.
Have fun, whatever you choose.