What should I do if I know of an SM who intentionally hurt themselves to be sent home because their bf/gf was threatening to leave them?

First u MUST COVER your ASS. Your must be able to prove what went on !! 2 It has to go up the chain of command . You my be able to bypass the Co But it has to go to the Bn. or Regiment. You could go to the IG. But your ducks better be in gear. Because shit rolls down hill ,& your are at the bottom.

This'll be a commander's call, ultimately.
Were I in the CP, I would call into question of any self-inflicted gunshot wound, ND or not. This is a Soldier who was supposed to deploy... presumably armed and with ammo. That definitely should receive scrutiny.
Think of it like this. If this had happened in Afghanistan, do you think it would have been investigated? Of course it would. And if the findings were EITHER it was an ND or purposeful (it was certainly one of the two), this bubba is going to be in trouble and called on the carpet to see a GO.
Since the Soldier is Guard, there is next to no chance a reasonable commander would make a finding that this was incurred in the line of duty, so I think we can put to rest that he will get military or VA disability. Social Security is another matter.
As for characterization of discharge, that should come down to a finding (with the help of police) that it was or was not a SIGW. If it was, OTH. If it wasn't, depends on the commander.

As for what you should do, my advice would be to share on an informal basis with your CoC what you KNOW and with an asterisk, what you THINK. Be sure to clearly define which is which. Then your job is done here, unless you are questioned during a formal 15-6 investigation. If questioned as a part of that, only share what you KNOW on any sworn statement or testimony.
Don't MSU.
Don't assume.
Don't guess.

Did said Soldier tell you they shot themselves in the foot? The reason why I ask this, is because if they did not, and you heard it through the "grapevine" and report it, then it's basically he said/she said scenario. An investigation would probably occur and the Soldier would probably have a hard time explaining how that happened. Also, what proof do you have to know that the unit Commander knows but is doing nithing?
BUT, if you were told directly by the Soldier in question that they shot intentionally shot themselves in the foot......

Situation #1) If you are hearing this not directly from the source, you can report it but tread carefully.

Situation #2) If you did hear this directly from the source, then you NEED to report this to higher authorities.