If he sold the business in 2013 he would have a loss in that year equal to the remaining basis.  I assume you are now working on his 2013 return rather than a 2017 or 18 return?

LHI's are generally MACRS 39 year property.  My memory about 2012 is hazy, but I don't *think* § 179 or bonus was available.

I looked it up, and both were valid options for Leasehold Improvement Property.  I think they are usually 15 year property, but the TCJA sort of messed that up (until a correction is made).

With that being said, if this is a Schedule C (or F), I would think Bonus would be best because it will save SE tax in 2012, and the depreciation recaptured would not be subject to SE tax in 2013 (unlike Section 179 recapture).

Seriously ?  It was available that long ago? Just one more reason I have doing prior year returns :-).  Sigh, the memory isn't what it used to be.
I agree, bonus is better to avoid the recapture issue.

BUT can I amend 2012 if it was already filed?
What date was the LHI put into service?
FYI https://www.irs.gov/pub/irs-prior/p946--2012.pdf
Maybe @joe7230 can join in the conversation