She didn't say this money was from illegals. However, to address that chance, a good revenue raiser might be to have banks require a green card or proof of citizenship (birth cert) or naturalization papers before money can be wired outbound for no cost. If these documents are not present, the US govt could charge $10 per $100 wired. The proceeds could fund the fence.
I like the idea but there needs to be a fine in there somewhere for sending money that you didn't declare taxes on, such as your undocumented neighbor.