May I echo the sentiments above; please tell your friend NOT TO PANIC. Their situation is the same as tens of thousands of other 'Accidental Americans' around the world going through the exact same experience at this time.
I'll also echo, to some degree, Guya's comment; the bank has discovered your friends tie to the US. Whether your friend replies or not, the bank will report an American tie to the IRS. There is now nothing that can be done on that front as your friend does not have a CLN, or a relinquishment argument(?). But, your friend does maintain control as to how they will respond from here forwards to the situation they now find themselves in.
The bank forwards that information to the IRS once yearly. They will forward it whether they have a US SSN or not.
Now, not is all as bad as the above would indicate.
Assuming your friend does not wish to simply ignore the situation (some do), the bank account is the first issue. There's nothing your friend or the bank can do if there is an unknown SSN. Simply tell the bank they are willing to sign a W-9 form (and NOT a W-8) admitting the US tie and once the SSN is rediscovered, they will supply it. The bank is only interested in identifying Americans. Once identified, the bank has no other interest other than the yearly reporting. The bank always retains the W-9 form in their records and it is not sent to the IRS.
The bank should be satisfied with this and should take no further action towards any account if it is the UK account of a UK resident. In some countries, the reaction might be different, and it is those situations which one often hears about; but in the UK, actual account closures are unusual (but, unfortunately, some do happen).
Be wary of the US accountant. You have mentioned your friend has joined a "Streamlined" programme, but for 5 years. The only streamlined programme your friend should contemplate is the Streamlined Foreign Offshore Procedures. This procedure only requires 3 years of past returns, and 6 years of past FBARS. There are other methods of coming into compliance, but those are for the professional advisor to discuss.
The US tax situation does not, usually, involve paying tax to the US. 82% of filers do not pay US tax. The difficulty, as Guya is identifying, is staying in compliance. Anything to do with the UK is 'foreign' to the US, and the US treats any foreign asset punitively. Once your friend enters the US tax system, they will have to carry on, each and every year until they die, and then some(!). There can be no further slip-ups. This leads many Accidental Americans to renounce their US citizenship (which DOES require 5 years of past filings). IMPORTANT QUESTION: Does your friend ever intend to return to the US to live? Do they wish to retain a US passport given their (business and other) circumstances and the requirement for accurate yearly US reporting?
EDIT TO ADD: There is no need for streamlined to be completed by any certain time. It will take as long as it takes, and it is unlikely the IRS will be taking any immediate action, even with the bank notification. They are now receiving millions(?) of these account reports from around the world. The only time constraint is the 31 days by the bank, and even that is questionable. DON'T PANIC, and don't allow others to attempt to panic your friend, even some tax advisors. The IRS is aware these unfortunate situations exist, although prolonged inaction (years) could have unpleasant consequences.