Calculating streamlined penalty for mother who screwed up reporting

I’m at my wits end trying to assist my US/UK citizen mother on her tax issues. I’m not a tax pro but I know more than most, and have two CPA friends to assist. However none of us have an answer here or any path to clarification. Before anyone asks, my mother had been working with two supposed international tax experts, both who fired her since they didn’t know how to deal with this issue.

My mom doesn’t have a lot of money but does have some retirement and savings funds she never reported on any of the information forms. One is her UK defined benefit pension from a while ago. We can find zero guidance on how this should be valued for penalty purposes and none of the advisors or CPA’s know. It pays out around 18k per year.

One of my CPA friends said the “conservative” thing to do is just pay the penalty on the 18k, but she puts this money into a bank account, meaning she will be penalized on the same income twice. Does anybody know where to begin to start here?