As always in medicine, there is no single answer to that. The more modern machines have a bore size of 70cm, which is comfortable compared to the 52 of older machines. That reduces claustrophobic fellings quite significantly.
In addition, depending on the field of view to be covered, a feet first position could be possible, thus leaving the head out of the machine. Your friend should just ask.
She will get a small ball handed over, which is a call device for the technicians. So she will not be left without contacting possibility (which also helps a lot for many patients). Many services also offers to play the patients own music, so she should bring some music she is feeling well with.
With friendly care and taking some time for the patient one can reduce the rate of claustrophobic drop outs from 5-10% of patients to a small single digit percentage, so if she tells them early, a good service should take time and keep the time pressure away to allow your friend to settle in.
Overall, it is painless, but quite boring (at least when I was in there for some scientific stuff for almost 3 hours).