Did you mean for them to get together? Male rabbits can breed as early as 10 weeks of age.
Is there a reason the female rabbit isnt fixed? At four years old her chance of female cancers is like 85%. If she could have it the owner risks it getting into other organs as well if it metastasizes. It can spread to their lungs, and every other vital organ.
I think you will just have to wait and watch. Gestation period is only 31 days so in one month you will see babies if she is pregnant. The female bunny in a few weeks will begin pulling fur from her tummy to make a nest for her babies and her stomach will of course grow in size. Female rabbits can also get pregnant again within 30min of giving birth. There is the option of aborting the babies during a spay if the owner chooses. I think most vets will do this if the owners wants it-not suggesting it, just letting you know.
Dont let them together again and maybe let your friend know she really should get her female bunny spayed and not let her bunny around anymore unaltered males.