Your lawyer will want to retain a good divorce lawyer, and can expect to spend a chunk of change on accountants - and possibly also investigators in the foreign nation(s) involved, depending on how complicated this matter is.

If the overseas assets are part of the marital estate, then she's entitled to her share. If it were me, I would focus on trying to get a domestic court to award her the domestic assets rather than having to start and litigate ancillary actions in another country (or countries) if her husband doesn't cooperate with the division of those assets.

Your mother needs to discuss the tax situation with an accountant or lawyer. Sometimes the divorce will proceed without mention of the tax issues; at other times, they become a significant complication, particularly if the IRS becomes involved. Whatever happens, if it were me, I would want some form of indemnification provision in the final judgment in case the IRS decides that he owes a lot of back taxes and (for the same reasons I suggested above) decides that they should pursue the domestic assets to satisfy that obligation.