I think the 18i20 might be a little overkill! 18 inputs and 20 outputs when all he wants it for is to plug his bass guitar into.

I'm running the 2i4 here, great little unit that is perfect for plugging my guitars, bass, and keyboards into. Plus a mic the odd times I do that.

Even that is probably more than he needs, the 2i2 or even the solo with one input would be fine.