at least a condom plus spermicide. I would get condoms that have spermicide in the lubricant as well as spermicide for her to insert before sex as well. And if you want to be EXTRA safe, she could use a diaphragm on top of that as well.
timalkin is correct of course, while I would be relatively confident in condom+spermicide, and even moreso in condom+diaphragm+spermicide, if you REALLY want to be sure, the best thing to do is just wait. If you're in a serious relationship, it's not like she's going anywhere
Let's not jump on the guy here...there are plenty of possible reasons his girlfriend is not on the pill, and that's not what he asked. He asked what other steps they could take, so let's answer and be nonjudgemental. He's clearly aware of the benefits of the pill and, for whatever reason, they have chosen not to use it.