So 29% business use when sold.  29% x $22,000 original cost = $6,380.  Subtract $1,774 from that and the basis is $4,606.  Trade-in allowance was $6,500, 29% of that is $1,885.  So you have a loss of $4,606 minus $1,885, or $2,721.  

I'm not sure I would try to get away with claiming a loss, but I wouldn't go out of my way to find a gain.  I tried to find some IRS guidance on this, not that it would be dispositive but could be informative.  Nothing.  I think they stepped in it when they banned considering it a like-kind exchange.  Maybe blame that on Congress.  

I would probably say that the day he traded it in, he didn't have any business mileage, so the basis of the 100% personal-use car was $22,000 less $1,774.  And since it's a personal asset, no loss allowed.  

Added:  Reading the original post, you say 48% business use in year of sale.  But same method applies.  And I note that it was not a trade-in, it was actually sold.  But trade-ins count as sales these days, anyway.  He didn't drive it for business the day he sold it, right? So it was a 100% personal vehicle.