Who is required to file a self-assessment

Asking for a friend, not me.    If someone did not work during 2017/18, but cashed out entirely an IRA that was a value less than $10,000 (so, about, what, 7 or 8K pounds?), they are well below the personal allowance for 2017/18 and wouldn't owe tax on it. Do they still need to file a self-assessment and report this income?The bolded bit from the HMRC website below seems to say so:

You’ll need to send a tax return if, in the last tax year:

    your income from self-employment was more than £1,000 - this is your ‘trading allowance’
    you got more than £2,500 from renting out property - contact the helpline if it was between £1,000 and £2,500
    you got more than £2,500 in other untaxed income, for example from tips or commission
    your income from savings or investments was £10,000 or more before tax - this includes money from bare trusts or interest in possession trusts
    your income from dividends from shares was £10,000 or more before tax - tell HMRC if it was over your dividends allowance
    you made profits from selling things like shares, a second home or other chargeable assets and need to pay Capital Gains Tax
    you were a company director - unless it was for a non-profit organisation (such as a charity) and you did not get any pay or benefits, like a company car
    your income (or your partner’s) was over £50,000 and one of you claimed Child Benefit
    you had income from abroad that you needed to pay tax on
    you lived abroad and had a UK income
    your taxable income was over £100,000
    you were a trustee of a trust or registered pension scheme
    you had a P800 from HMRC saying you did not pay enough tax last year - and you did not pay what you owe through your tax code or with a voluntary payment
    your State Pension was more than your Personal Allowance and was your only source of income - unless you started getting your pension on or after 6 April 2016

Asking for a friend, not me.    If someone did not work during 2017/18, but cashed out entirely an IRA that was a value less than $10,000 (so, about, what, 7 or 8K pounds?), they are well below the personal allowance for 2017/18 and wouldn't owe tax on it. Do they still need to file a self-assessment and report this income?

If there’s no tax that needs to be paid and no tax credits that need to be claimed, what would be the point of filing a return?  What is the friend hoping to achieve by filing a return?

Compliance with reporting requirements. The HMRC website language seems to indicate that if you have over 2,400 of income you need to file.

Generally, if you have foreign source income and are on the arising basis,you need to file a return, even if no tax is due. Your friend could call HMRC, which may or may not be answered by someone who actually knows. Alternative, take the way easier route of doing a SA, as no tax will be due anyway if there was no other income to report.

Compliance with reporting requirements. The HMRC website language seems to indicate that if you have over 2,400 of income you need to file.

The UK system is different from the US system.  You’re required to report and pay tax on income you need to pay tax on (i.e. income that hasn’t already had the tax withheld before you get it - for example, cash-in-hand payments like tips, or foreign income).

Most people in the UK only have income that’s either tax-free or has had the tax withheld before they get it.  So most people in the UK don’t need to file SA1 and in fact probably never heard of SA1.

It’s in the section you quoted  - “foreign income you need to pay tax on”  If the recipient doesn’t need to pay tax on the foreign income, filing a return would be pointless, as far as I can see. 

Your friend could call HMRC, which may or may not be answered by someone who actually knows.

Yes, if your friend rings HMRC and asks to be sent a paper return to complete, a discussion is likely to ensue about whether that’s really necessary.  And that will answer your friend’s question.  (They don’t mind if you file online, whether you need to or not, but filing unnecessary paper copies creates extra work.)

So, the response so far is nobody actually knows.

I'll pass that along, thanks.

So, the response so far is nobody actually knows.

I'll pass that along, thanks.

There’s no consequences whatever your friend does. 

File an online return to report that the income has been received, don’t bother since no tax is due, or ring for reassurance.  The outcome is the same. 

So your friend just needs to decide which of the three possible options to go for.