About the whole marriage thing, though, the complicated part would be if she crossed the border & had no passport, visa, etc, basically a EWI (Entered Without Inspection).
Then the marriage plan might not be the best idea, cuz if she's an EWI that would require her to leave the country before she can get residency through a marriage, and the process is VERY complex and VERY long.
Your son would have to have a "proof of hardship", basically something that proves WHY he needs her back in the country & why he can't go live with her instead. <-- very hard to do.
If by chance he is successful in his proof of hardship, she'll still have to wait until the whole process is approved until she can come back, & that might take up to a year or god knows how many more.

Good proofs of hardship would be things such as...
- He's got a sick family member that only HE can care for, & therefore cannot leave the country to stay with his wife.
- He's got children with his wife that would somehow suffer in their growing process if they left the country (i.e. if her country lacks good education, etc)
- He's got children that are emotionally attached to her & also cannot leave the country
- He himself is too sick or has some sort of illness or disability that would make him unable to travel, or she's the only one that can care for him, etc

Well, you get the gist. Those are EXCELLENT proofs of hardships, but there are others that are more realistic for your case, too. There's a thread here somewhere that lists a bunch of them...

Anywho, if she's an EWI, you might wanna reconsider the whole marriage idea, cuz she might end up not being able to come back. =/