Technically, you are not obligated to do anything in this situation. Whether the issue is partly your fault or not is irrelevant.
It is very nice of you to email suggestions to them, though. If it really comes down to the point where they ask you to come back to the office to help solve this issue, you might try to just negotiate a temporary contract, wherein you could help them out as a side-job for a couple days or weeks, when possible outside of your new job. You'd have to be careful about that though - your new employer may have rules against this, so you would have to talk to them first. For that matter, I don't know what country you are in - something like this might be fine in one country, but there might be rules about it in some other country.
But again, you are under no obligation to do any of these things. Sounds like a harsh way to put it, but it's true.
Another thing to consider is the relationship with your previous employer. I like his suggestion of trying to help out, especially if it is not much effort. As he suggests, it is possible that refusing to help out (when you are both able, and contractually allowed to do so) could sour the excellent relationship that you have with your previous employer. It could turn a good reference into a bad one.
Consider helping out if you are able, especially if it is not much effort, and if you are contractually allowed to by your new employer. I've been in the same situation, and it worked out well for all parties involved.