This is clearly a real mess. I have to say that, ultimately, you are responsible for your own tax affairs. Tax Returns have to be signed by the taxpayer (you) so I wonder why you didn't ask why you were not being given the forms to sign and why you were not receiving tax demands at your home or business address given that you appear to be self-employed. Even if the business is run through a company, there may have been corporation tax to pay.
You should always read letters about your own tax affairs whether you have somebody acting for you or not. In my 35 years in tax, people seem to have a blind spot when it comes to tax when the rest of their financial affairs are in order so I have seen all this before and I still don't understand it.
Without sight of the paperwork, I cannot tell how the £10,000 is made up or how many years are involved. As I said in my previous post, it is probably be a combination of tax, NIC, late tax return penalties, tax surcharges and interest. Read about penalties here
You mention paying a monthly amount. Why? You probably don't pay any other bill you get without making sure its correct so why do it with tax? You need to find out what has been going on, exactly what you are being asked to pay and if it is correct which I doubt and I'm afraid it will involve dropping your friend in the proverbial. My understanding is that HMRC employees are not allowed to get involved with doing tax work on a private basis.
The only way you are going to have a hope of getting some of the penalties reduced or even cancelled is to make a complaint to HMRC and tell them why you are in the situation you are in and that is down to one of their employees not doing what they said they would do for you. You can make a complaint here
. Contact HMRC by telephone. I can assure you that if what you have told me is true, there will be a firestorm in HMRC.
You should also employ an accountant or tax adviser who is not a friend to clear the mess up and get your tax returns and whatever else is latye up to date. You clearly have no desire to complete your tax returns yourself so pay somebody who knows what they are doing to do it. If I wasn't a tax consultant, had little tax knowledge and I ran a business, I'd employ an accountant in the same way I call a plumber if I have a water leak. At least if things go wrong, you can sue.
If after this mess is cleared, you have tax and penalties and interest to pay, a decent accountant or tax adviser can assits you in negotiating a payment plan with HMRC and ensure that your rights are protected.
I hope this helps but let me know if you have any further questions.