What will the IRS do?

Call the IRS and correct the mistake.

this is something that IRS matches, it is a surprise it has gone on for 18 yrs and not been noticed, but 1040x's are the way to correct them

The IRS won't do anything because they don't know about it. Your family relative should have reviewed it to make sure the personal details were correct, at least the first time. This will need correcting if your family relative is to receive future benefits that he/she is entitled to in the future. She will have to hire a tax lawyer to have every past tax return corrected. Whether it requires a complete new return or not, I don't know but, the IRS will likely want to verify who he/she is before they will fix the error. It won't happen overnight.

She will need to contact the IRS & see what they require to correct the error. It will be a difficult & tedious process.

Probably not much will happen. Her husband's SSN is the primary tax record ID. But I would suggest she talk to the IRS to find out what implications, if any, might be. This would not be an illegal or criminal event. Just an error on the tax forms. Also, the IRS doesn't go back more than about 3 years, 7 years if fraud is suspected. Did your family relative have any earned income? I suspect if the IRS found the error (name and SSN mismatch), they would have sent some letters.

call the IRS.. they'll possibly make you do amended returns for the last 7 years. Moreover she should be concerned that her social security income numbers won't be under her number which will great affect her benefits at retirement. It could say she had zero income for those years to base benefits on.

The returns should have been rejected by IRS if the first four letters of the last name does not match the Social Security Number but unless she had self-employment income it won't make any difference.

That's very strange since a SSN mis-match would result in the e-filed return being rejected.

She is required to check.

She needs to contact the IRS and try to sort out the mess.