There is a lower rate of Jobseeker's Allowance (JSA) for those under age 25 but she would not be prevented form claiming if dismissed for misconduct or if she left voluntarily. Being sacked does not automatically mean there was misconduct by the way. Someone could be dismissed because of the level of sickness so that is not misconduct and there are other circumstances that may appear to be misconduct which are not for the purposes of JSA.
Whether it's misconduct or leaving voluntarily (LV), if there's a doubt, the case will be sent to a Decison Maker.
For LV, the claimant needs to show there was just cause (a good reason) for doing so and the Decision Maker will need to know why the person could not wait until alternative employment had been found (why was it so urgent to leave when they did) and what efforts had been made to find another job.
Although the JSA can be sanctioned for up to twenty-six weeks, I would always suggest folk make a claim for JSA rather than not claim for that period. The JSA is paid until a decision on the misconduct or LV is made and it is always stopped for a future period so it's not a case of money having to be re-paid and it could be stopped for a period less than twenty-six weeks.