How big are the holes? Is there enough metal there to hold the bolts if you enlarge the holes? Assuming there is, then the tool you want is not a drill, but a reamer.
I suggested drill because (assuming he gets the correct size) the walls of the hole should be square to the chainring. While this is not guaranteed with a drill[1], it's guaranteed NOT with a tapered reamer. One end of the hole (or both, if you go from both sides) will be too big when the other (or middle) just fits; and considering the application and material such a mount will wear quickly.
As well, using such a reamer takes some skill. It's all to easy to make a hole that is not round at all, but a multiply-lobed hole, one lobe for each of the reamer edges, and of course, it is a lot easier to make the hole too big.
[1] Holes made by drills are not guaranteed to be any of the following:
a) round
b) straight
c) the same shape all the way through
d) the size you want them to be
e) where you want them to be