Are all these calendars from her iCloud account, or are any from another source such as Gmail?
New events are not added randomly to calendars. They are assigned to a default calendar, and when creating a new event a choice has to be made if the event is to go into a different calendar. Your friend's event's would 'disappear' only if she had set the Calendar app on her device not to display that particular calendar.
Merging calendars can’t be done as such but the events in an unwanted calendar can be moved to a preferred calendar. This will involve some manual work, and it depends too on what she wants to achieve. Does she want just one calendar, or does she intend to use a couple - such as Work, and Home? Or more?
If she had a computer with which she could export a calendar's events, and then delete the unwanted calendar before importing the exported calendar's events back into a desired calendar, but you seem to indicate that she does not have a computer and this would not be possible otherwise. Neither can it be done in the Calendar application at icloud.com, which lacks this capability.
Therefore manual editing in an iOS device must be done. What I would do in order to maintain control of each event is to hide all but one non-preferred calendar and work on events in that calendar, going through future dates until she feels that all the events in that calendar have been moved to the preferred calendar.
Moving events can be done by editing the event, finding the 'Calendar' label, then use the toggle to select a different calendar for that event. It should then disappear from view and into another calendar which, remember, is not being displayed at this point.
It’s a bit of work but in the end it should have a satisfactory result.
