It's hard to say Bandai is making a really tough decision, they could roll back but that would anger and F2P/Android player or people unaware of the bug that has rolled and gotten characters they needed.

They could try to charge people for the gems which I highly doubt they will do as it is there fault and the gems were listed for 120 yen. It's not like they were listed otherwise it's like putting 1$ on something in a store getting to the cash register and them trying to be like we actually want 3$.

The last thing I can think of is just removing any of the gems bought from the people who abused the cheap gem bug from there account. It would be a bit lame (even though I am jealous I play Android) it would be way to big of a loss to just let players that spend tons of money especially on the most profitable day of the year and say F it we don't care keep your 3k of gems.

The best scenario is Bandai pretends nothing happened like 7/11 gives us free gems and acts like nothing went wrong.