when a USA citizen marries in foreign country

[okatay]

if a US citizen marries in foreign country, with a foreign person

and if he files his tax return as married filling separately

then the foreign spouse is totally out of the equation from US tax system and no paperwork for her? and all the US spouse must do is to mark "married fililng separately" and that is all? the rest is same as if he is filing single (yes tax rate might be different but as far as what he does when completing return)?

Or, still there will be something, some paperwork for the foreign spouse to USA, just because she marries US citizen but has nothing else to do with US and totally lives and works abroad? if so is that form, whatever it is, is supported  here?

[Critter]

All the US citizen has to do is file separately (Single is NOT allowed if you are married).

If you are a US citizen and your spouse does not have a Social Security number or an ITIN and you are not applying for an ITIN with the tax return then you can only file your tax return as Married Filing Separately.  Where asked to enter the spouse's Social Security number enter 999-88-9999.  You can only print and mail your tax return; it cannot be e-filed.  When you print the tax return erase the Social Security number for your spouse and manually enter NRA for non-resident alien.

See this TurboTax support FAQ for the procedure to print and mail a tax return using the online editions - https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online

If you are filing your tax return as Married Filing Jointly you must apply for an ITIN with the tax return and you would need to be providing a statement with your tax return that you want your Nonresident Alien Spouse Treated as a Resident.  See IRS Publication 54 Tax Guide for U.S. Citizens and Resident Aliens Abroad pages 6 and 7 - https://www.irs.gov/pub/irs-pdf/p54.pdf

Go to this IRS website for ITIN information - https://www.irs.gov/individuals/international-taxpayers/general-itin-information